<PAGE>   1

     As Filed With the Securities and Exchange Commission on June 15, 1998
                                                   Registration No. 333-________
================================================================================

                       SECURITIES AND EXCHANGE COMMISSION
                             WASHINGTON. D.C. 20549

                            -----------------------

                                    FORM S-8
                             REGISTRATION STATEMENT
                                      UNDER
                           THE SECURITIES ACT OF 1933

                            -----------------------

                           SONUS PHARMACEUTICALS, INC.
             (Exact name of registrant as specified in its charter)

            DELAWARE                                  95-4343413
(State or other jurisdiction of                     (I.R.S. Employer
 incorporation or organization)                    Identification No.)

        22026 20th Avenue S.E., Bothell, Washington       98021
         (Address of Principal Executive Offices)       (Zip Code)

                            -----------------------

              INCENTIVE STOCK OPTION, NONQUALIFIED STOCK OPTION AND
                      RESTRICTED STOCK PURCHASE PLAN -1991
                            (Full titles of the plan)

                            -----------------------

                     Steven C. Quay, M.D., Ph.D., President,
                      Chief Executive Officer and Secretary
                           Sonus Pharmaceuticals, Inc.
                22026 20th Avenue S.E., Bothell, Washington 98021
                     (Name and address of agent for service)
                                 (425) 487-9500
          (Telephone number, including area code, of agent for service)


                                    Copy to:
                                K.C. Schaaf, Esq.
                            Christopher D. Ivey, Esq.
           Stradling Yocca Carlson & Rauth, a Professional Corporation
      660 Newport Center Drive, Suite 1600, Newport Beach, California 92660
                                 (949) 725-4000

                         CALCULATION OF REGISTRATION FEE
<TABLE>
<CAPTION>
===============================================================================================
Title of Securities    Amount To Be     Proposed Maximum    Proposed Maximum      Amount of
 To Be Registered    Registered(1)(2)       Offering           Aggregate       Registration Fee
                                         Price Per Share     Offering Price
===============================================================================================
<S>                  <C>                 <C>                 <C>                <C>
  Common Stock,
  $0.001 par value    400,000 shares        $11.75(3)        $4,700,000(3)       $1,386.50
===============================================================================================
</TABLE>

(1) Includes additional shares of Common Stock that may become issuable pursuant
    to the anti-dilution adjustment provisions of the Incentive Stock Option,
    Nonqualified Stock Option and Restricted Stock Purchase Plan - 1991 (the
    "1991 Plan").

(2) 546,017 shares of Common Stock available for grant under the 1991 Plan were
    registered on a Registration Statement on Form S-8 on December 15, 1995
    (Registration No. 33-08623) and 774,810 shares of Common Stock available for
    grant under the 1991 Plan were registered on a Registration Statement on
    Form S-8 on September 22, 1997 (Registration No. 33-36093).

(3) The aggregate offering price of 400,000 shares of Common Stock registered
    hereby which would be issued upon exercise of the remaining options which
    Registrant is authorized to issue under its 1991 Plan is estimated solely
    for purposes of calculating the registration fee, in accordance with Rule
    457(h), on the basis of the price of securities of the same class, as
    determined in accordance with Rule 457(c), using the average of the high and
    low price reported by the Nasdaq National Market for the Common Stock on
    June 12, 1998, which was $11.75 per share.

                               Page 1 of 6 Pages
                            Exhibit Index on Page 4


<PAGE>   2

                                     PART II
               INFORMATION REQUIRED IN THE REGISTRATION STATEMENT

Item 3.  Incorporation of Documents by Reference.

         The contents of the Registrant's Registration Statement on Form S-8
(Registration No. 33-80623) are incorporated herein by reference.

Item 8.  Exhibits.

         The following exhibits are filed as part of this Registration
Statement:

        Number               Description
        ------               -----------
         5.1          Opinion of Stradling, Yocca, Carlson & Rauth, a
                      Professional Corporation, Counsel to the Registrant.

        23.1          Consent of Stradling, Yocca, Carlson & Rauth, a
                      Professional Corporation (included in the Opinion filed as
                      Exhibit 5.1).

        23.2          Consent of Ernst & Young LLP, Independent Auditors.

        24.1          Power of Attorney (included on signature page to the
                      Registration Statement at page S-1).



                                      II-1

<PAGE>   3

                                   SIGNATURES

         Pursuant to the requirements of the Securities Act of 1933, the
Registrant certifies that it has reasonable grounds to believe that it meets all
of the requirements for filing on Form S-8 and has duly caused this Registration
Statement to be signed on its behalf by the undersigned, thereunto duly
authorized, in the City of Bothell, State of Washington, on the 15th day of
June, 1998.


                                       SONUS PHARMACEUTICALS, INC.


                                       By: /s/ Steven C. Quay
                                           -------------------------------------
                                           Steven C. Quay, M.D., Ph.D., 
                                           Chief Executive Officer, President 
                                           and Secretary


                                POWER OF ATTORNEY

         We, the undersigned officers and directors of SONUS Pharmaceuticals,
Inc., do hereby constitute and appoint Steven C. Quay and Gregory Sessler, or
either of them, our true and lawful attorneys-in-fact and agents, each with full
power of substitution and resubstitution, for him and in his name, place and
stead, in any and all capacities, to sign any and all amendments to this
Registration Statement, and to file the same, with exhibits thereto, and other
documents in connection therewith, with the Securities and Exchange Commission,
granting unto said attorneys-in-fact and agents, and each of them, full power
and authority to do and perform each and every act and thing requisite or
necessary to be done in and about the premises, as fully to all intents and
purposes as he might or could do in person, hereby ratifying and confirming all
that each of said attorneys-in-fact and agents, or his substitute or
substitutes, may lawfully do or cause to be done by virtue hereof.

         Pursuant to the requirements of the Securities Act of 1933, this
Registration Statement has been signed by the following persons in the
capacities and on the date indicated.


<TABLE>
<S>                                  <C>                                         <C>
/s/ Steven C. Quay                   Chief Executive Officer, President,         June 15, 1998
---------------------------------    Secretary and Director (Principal
Steven C. Quay, M.D., Ph.D.          Executive Officer)


/s/ Gregory Sessler                  Chief Financial Officer (Principal          June 15, 1998
--------------------------------     Financial and Principal Accounting
Gregory Sessler                      Officer)


/s/ George W. Dunbar, Jr.            Director                                    June 15, 1998
--------------------------------
George W. Dunbar, Jr.


/s/ Harry A. Shoff                   Director                                    June 15, 1998
--------------------------------
Harry A. Shoff


/s/ Dwight Winstead                  Director                                    June 15, 1998
--------------------------------
Dwight Winstead
</TABLE>


                                      S-1

<PAGE>   4
                                  EXHIBIT INDEX

<TABLE>
<CAPTION>
Exhibit                                                                          Sequential
Number                 Description                                               Page Number
-------                -----------                                               -----------
<C>                    <S>                                                       <C>
    5.1                Opinion of Stradling, Yocca, Carlson & Rauth, a                5
                       Professional Corporation, Counsel to the Registrant.

   23.1                Consent of Stradling, Yocca, Carlson & Rauth, a                5
                       Professional Corporation (included in the Opinion filed
                       as Exhibit 5.1).

   23.2                Consent of Ernst & Young LLP, Independent Auditors.            6

   24.1                Power of Attorney (included on signature page to the         S-1
                       Registration Statement at page S-1).
</TABLE>
[{"type": "S-8", "sequence": "1", "description": "FORM S-8", "secsgml_size_bytes": 9626}]
{"company-data": {"conformed-name": "SONUS PHARMACEUTICALS INC", "cik": "0000949858", "assigned-sic": "2835", "irs-number": "954343413", "state-of-incorporation": "DE", "fiscal-year-end": "1231"}, "filing-values": {"form-type": "S-8", "act": "33", "file-number": "333-56933", "film-number": "98648975"}, "business-address": {"street1": "22026 2OTH AVE SE", "street2": "STE 102", "city": "BOTHELL", "state": "WA", "zip": "98021", "phone": "2064879500"}, "mail-address": {"street1": "22026 20TH AVENUE SE, SUITE 102", "street2": "22026 20TH AVENUE SE, SUITE 102", "city": "BOTHELL", "state": "WA", "zip": "98021"}}
1

     As Filed With the Securities and Exchange Commission on June 15, 1998
                                                   Registration No. 333-________
================================================================================

                       SECURITIES AND EXCHANGE COMMISSION
                             WASHINGTON. D.C. 20549

                            -----------------------

                                    FORM S-8
                             REGISTRATION STATEMENT
                                      UNDER
                           THE SECURITIES ACT OF 1933

                            -----------------------

                           SONUS PHARMACEUTICALS, INC.
             (Exact name of registrant as specified in its charter)

            DELAWARE                                  95-4343413
(State or other jurisdiction of                     (I.R.S. Employer
 incorporation or organization)                    Identification No.)

        22026 20th Avenue S.E., Bothell, Washington       98021
         (Address of Principal Executive Offices)       (Zip Code)

                            -----------------------

              INCENTIVE STOCK OPTION, NONQUALIFIED STOCK OPTION AND
                      RESTRICTED STOCK PURCHASE PLAN -1991
                            (Full titles of the plan)

                            -----------------------

                     Steven C. Quay, M.D., Ph.D., President,
                      Chief Executive Officer and Secretary
                           Sonus Pharmaceuticals, Inc.
                22026 20th Avenue S.E., Bothell, Washington 98021
                     (Name and address of agent for service)
                                 (425) 487-9500
          (Telephone number, including area code, of agent for service)

                                    Copy to:
                                K.C. Schaaf, Esq.
                            Christopher D. Ivey, Esq.
           Stradling Yocca Carlson & Rauth, a Professional Corporation
      660 Newport Center Drive, Suite 1600, Newport Beach, California 92660
                                 (949) 725-4000

                         CALCULATION OF REGISTRATION FEE
===============================================================================================
Title of Securities    Amount To Be     Proposed Maximum    Proposed Maximum      Amount of
 To Be Registered    Registered(1)(2)       Offering           Aggregate       Registration Fee
                                         Price Per Share     Offering Price
===============================================================================================
Common Stock,
  $0.001 par value    400,000 shares        $11.75(3)        $4,700,000(3)       $1,386.50
===============================================================================================
(1) Includes additional shares of Common Stock that may become issuable pursuant
    to the anti-dilution adjustment provisions of the Incentive Stock Option,
    Nonqualified Stock Option and Restricted Stock Purchase Plan - 1991 (the
    "1991 Plan").

(2) 546,017 shares of Common Stock available for grant under the 1991 Plan were
    registered on a Registration Statement on Form S-8 on December 15, 1995
    (Registration No. 33-08623) and 774,810 shares of Common Stock available for
    grant under the 1991 Plan were registered on a Registration Statement on
    Form S-8 on September 22, 1997 (Registration No. 33-36093).

(3) The aggregate offering price of 400,000 shares of Common Stock registered
    hereby which would be issued upon exercise of the remaining options which
    Registrant is authorized to issue under its 1991 Plan is estimated solely
    for purposes of calculating the registration fee, in accordance with Rule
    457(h), on the basis of the price of securities of the same class, as
    determined in accordance with Rule 457(c), using the average of the high and
    low price reported by the Nasdaq National Market for the Common Stock on
    June 12, 1998, which was $11.75 per share.

                               Page 1 of 6 Pages
                            Exhibit Index on Page 4
2

                                     PART II
               INFORMATION REQUIRED IN THE REGISTRATION STATEMENT

Item 3.  Incorporation of Documents by Reference.

         The contents of the Registrant's Registration Statement on Form S-8
(Registration No. 33-80623) are incorporated herein by reference.

Item 8.  Exhibits.

         The following exhibits are filed as part of this Registration
Statement:

        Number               Description
        ------               -----------
         5.1          Opinion of Stradling, Yocca, Carlson & Rauth, a
                      Professional Corporation, Counsel to the Registrant.

        23.1          Consent of Stradling, Yocca, Carlson & Rauth, a
                      Professional Corporation (included in the Opinion filed as
                      Exhibit 5.1).

        23.2          Consent of Ernst & Young LLP, Independent Auditors.

        24.1          Power of Attorney (included on signature page to the
                      Registration Statement at page S-1).

                                      II-1
3

                                   SIGNATURES

         Pursuant to the requirements of the Securities Act of 1933, the
Registrant certifies that it has reasonable grounds to believe that it meets all
of the requirements for filing on Form S-8 and has duly caused this Registration
Statement to be signed on its behalf by the undersigned, thereunto duly
authorized, in the City of Bothell, State of Washington, on the 15th day of
June, 1998.

                                       SONUS PHARMACEUTICALS, INC.

                                       By: /s/ Steven C. Quay
                                           -------------------------------------
                                           Steven C. Quay, M.D., Ph.D., 
                                           Chief Executive Officer, President 
                                           and Secretary

                                POWER OF ATTORNEY

         We, the undersigned officers and directors of SONUS Pharmaceuticals,
Inc., do hereby constitute and appoint Steven C. Quay and Gregory Sessler, or
either of them, our true and lawful attorneys-in-fact and agents, each with full
power of substitution and resubstitution, for him and in his name, place and
stead, in any and all capacities, to sign any and all amendments to this
Registration Statement, and to file the same, with exhibits thereto, and other
documents in connection therewith, with the Securities and Exchange Commission,
granting unto said attorneys-in-fact and agents, and each of them, full power
and authority to do and perform each and every act and thing requisite or
necessary to be done in and about the premises, as fully to all intents and
purposes as he might or could do in person, hereby ratifying and confirming all
that each of said attorneys-in-fact and agents, or his substitute or
substitutes, may lawfully do or cause to be done by virtue hereof.

         Pursuant to the requirements of the Securities Act of 1933, this
Registration Statement has been signed by the following persons in the
capacities and on the date indicated.
/s/ Steven C. Quay                   Chief Executive Officer, President,         June 15, 1998
---------------------------------    Secretary and Director (Principal
Steven C. Quay, M.D., Ph.D.          Executive Officer)

/s/ Gregory Sessler                  Chief Financial Officer (Principal          June 15, 1998
--------------------------------     Financial and Principal Accounting
Gregory Sessler                      Officer)

/s/ George W. Dunbar, Jr.            Director                                    June 15, 1998
--------------------------------
George W. Dunbar, Jr.

/s/ Harry A. Shoff                   Director                                    June 15, 1998
--------------------------------
Harry A. Shoff

/s/ Dwight Winstead                  Director                                    June 15, 1998
--------------------------------
Dwight Winstead
S-1
4
                                  EXHIBIT INDEX
Exhibit                                                                          Sequential
Number                 Description                                               Page Number
-------                -----------                                               -----------
5.1                Opinion of Stradling, Yocca, Carlson & Rauth, a                5
                       Professional Corporation, Counsel to the Registrant.

   23.1                Consent of Stradling, Yocca, Carlson & Rauth, a                5
                       Professional Corporation (included in the Opinion filed
                       as Exhibit 5.1).

   23.2                Consent of Ernst & Young LLP, Independent Auditors.            6

   24.1                Power of Attorney (included on signature page to the         S-1
                       Registration Statement at page S-1).