1


                                                                     Exhibit 5.1


                                  June 9, 1998


Warren Bancorp, Inc.
10 Main Street
Peabody, MA 01960


Ladies and Gentlemen:


         This opinion is furnished in connection with the registration, pursuant
to the Securities Act of 1933, as amended (the "Securities Act"), of 600,000
shares (the "Shares") of common stock, par value $.10 per share ("Common
Stock"), of Warren Bancorp, Inc., a Massachusetts corporation (the "Company"),
pursuant to the Company's 1998 Incentive and Nonqualified Stock Option Plan (the
"Plan").

         In connection with rendering this opinion, we have examined the
Articles of Organization and the Bylaws of the Company, each as amended; such
records of the corporate proceedings of the Company as we deemed material; a
registration statement on Form S-8 under the Securities Act relating to the
Shares (the "Registration Statement"), the Plan, and such other certificates,
receipts, records and documents as we considered necessary for the purposes of
this opinion.

         We are attorneys admitted to practice in the Commonwealth of
Massachusetts. We express no opinion concerning the laws of any jurisdictions
other than the laws of the United States of America and the Commonwealth of
Massachusetts.

         Based upon the foregoing, we are of the opinion that, when the Shares
have been issued and paid for in accordance with the terms of the Plan, the
Shares will be legally issued, fully paid and nonassessable shares of the
Company's Common Stock.

         The foregoing assumes that all requisite steps will be taken to comply
with the requirements of the Securities Act and applicable requirements of state
laws regulating the offer and sale of securities.

         We hereby consent to the filing of this opinion as an exhibit to the
Registration Statement.




                                        Very truly yours,


                                        GOODWIN, PROCTER & HOAR LLP