EXHIBIT 5.1

                       [COOLEY GODWARD LLP LETTERHEAD]

July 21, 2003


Onyx Pharmaceuticals, Inc.
3031 Research Drive
Richmond, CA 94806

Ladies and Gentlemen:

      You have requested our opinion with respect to certain matters in
connection with the sale by Onyx Pharmaceuticals, Inc., a Delaware corporation
(the "Company"), of up to 5,750,000 shares of the Company's common stock, par
value $0.001 (the "Shares"), including 750,000 shares of common stock for which
the underwriters have been granted an over-allotment option, pursuant to a
Registration Statement on Form S-3, a related Registration Statement on Form S-3
filed pursuant to Rule 462(b) under the Securities Act of 1933, as amended, and
the related Prospectus and Prospectus Supplement filed with the Securities and
Exchange Commission (the "Commission"). All of the Shares are to be sold by the
Company as described in the Registration Statements and related Prospectus and
Prospectus Supplement.

      In connection with this opinion, we have examined and relied upon the
Registration Statements and related Prospectus included therein, the Prospectus
Supplement filed with the Commission pursuant to Rule 424 under the Securities
Act of 1933, as amended, the Company's Amended and Restated Certificate of
Incorporation and Bylaws, as currently in effect, and the originals or copies
certified to our satisfaction of such other documents, records, certificates,
memoranda and other instruments as we deem necessary or appropriate to enable us
to render the opinion expressed below. We have assumed the genuineness and
authenticity of all documents submitted to us as originals, the conformity to
originals of all documents submitted to us as copies thereof and the due
execution and delivery of all documents where due execution and delivery are a
prerequisite to the effectiveness thereof.

      On the basis of the foregoing, and in reliance thereon, we are of the
opinion that the Shares, when sold in accordance with the Registration
Statements and the related Prospectus and the Prospectus Supplement, will be
validly issued, fully paid and nonassessable.

      We consent to the reference to our firm under the caption "Legal Matters"
in the Prospectus Supplement and the Prospectus included in the Registration
Statements and to the filing of this opinion as an exhibit to the Registration
Statements.

Sincerely,


COOLEY GODWARD LLP



/s/ Laura A. Berezin

Laura A. Berezin