EXHIBIT 5.1 A PARTNERSHIP INCLUDING PROFESSIONAL CORPORATIONS HINSHAW & CULBERTSON Belleville, Illinois 222 North La Salle Street San Francisco, California Bloomington, Illinois Chicago, Illinois 60601-1081 Ft. Lauderdale, Florida Champaign, Illinois Jacksonville, Florida Crystal Lake, Illinois 312.704.3000 Miami, Florida Joliet, Illinois _______ Tampa, Florida Lisle, Illinois Munster, Indiana Peoria, Illinois Telefax 312.704.3001 Minneapolis, Minnesota Rockford, Illinois St. Louis, Missouri Springfield, Illinois Appleton, Wisconsin Waukegan, Illinois March 2, 1998 Brookfield, Wisconsin Lake Geneva, Wisconsin Milwaukee, Wisconsin Writer's Direct Dial No. File No. (312) 704-3852 764359 VIA FACSIMILE & FEDERAL EXPRESS - ------------------------------- First Midwest Bancorp, Inc. 300 Park Boulevard, Suite 405 P.O. Box 459 Itasca, Illinois 60143-0459 RE: REGISTRATION STATEMENT ON FORM S-4 Ladies and Gentlemen: You have requested our opinion in connection with the above-referenced registration statement (the "Registration Statement") for the registration of up to 9,725,509 shares of Common Stock, $,01 par value per share, of the Company (the "Shares") in connection with the Company's acquisition of Heritage Financial Services, Inc. In arriving at the opinion expressed below, we have examined the Registration Statement and such other documents as we have deemed necessary to enable us to express the opinion hereinafter set forth. In our examination, we have assumed the authenticity of all documents submitted to us as originals, the conformity to the original documents of all documents submitted to us as copies, the genuineness of all signatures on documents reviewed by us and the legal capacity of natural persons. Based upon and subject to the foregoing, we are of the opinion that the Shares have been duly authorized and, when issued in accordance with the terms and conditions set forth in the Registration Statement, will be validly issued, fully paid and non-assessable. First Midwest Bancorp, Inc. March 2, 1998 Page 2 We hereby consent to the reference to our firm under the caption "Opinions" in the Registration Statement and to the use of this opinion as an exhibit to the Registration Statement. Very truly yours, /s/ Timothy M. Sullivan Timothy M. Sullivan TMS/mm