1

                                                                       Exhibit 5

                [LETTERHEAD OF MITCHELL SILBERBERG & KNUPP LLP]

                                  June 7, 2001


VIA EDGAR


Westcorp
23 Pateur Road
Irvine, California 92618

      Re:   Westcorp
            Registration Statement on Form S-8

Ladies and Gentlemen:

      We have acted as legal counsel for Westcorp (the "Company") in connection
with the preparation of the Company's Registration Statement on Form S-8 under
the Securities Act of 1933, as amended (the "Registration Statement"), to be
filed with the Securities and Exchange Commission, relating to 3,000,000 shares
of the Company's Common Stock issuable upon exercise of stock options (the
"Shares") granted or to be granted under the Company's 2001 Stock Option Plan
(the "Plan").

      In our capacity as counsel for the Company and for purposes of this
opinion, we have made those examinations and investigations of the legal and
factual matters we deemed advisable, and have examined the originals, or copies
identified to our satisfaction as being true copies of the originals, of the
certificates, documents, corporate records, and other instruments which we, in
our judgment, have considered necessary or appropriate to enable us to render
the opinion expressed below. In the course of our examinations and
investigations we have assumed the genuineness of all signatures on an original
document we have reviewed and on originals of documents where we have reviewed
copies, and the due execution and delivery of all documents requiring due
execution and delivery for the effectiveness thereof. We have also assumed that
the issuance of options pursuant to the Plan and entering into of stock option
agreements pursuant to the Plan have been, are and will be accomplished in
accordance with the terms of the Plan.

      Based upon and subject to the foregoing and in reliance thereon, and
subject to the assumptions, exceptions and qualifications set forth herein, it
is our opinion that the


   2

Westcorp
June 7, 2001
Page 1


Shares will be, when issued in compliance with the Plan and upon receipt of full
consideration payable therefor, legally issued, fully paid and non-assessable.

      We consent to the filing of this opinion with the Registration Statement.
This opinion is given as of the date hereof and we assume no obligation to
advise you of changes that may hereafter be brought to our attention.


                                Very truly yours,


                               /s/ Andrew E. Katz

                                 Andrew E. Katz
                                       for
                         MITCHELL SILBERBERG & KNUPP LLP