EXHIBIT 5.1


               [FAIR, ISAAC AND COMPANY, INCORPORATED LETTERHEAD]



                                        December 17, 1997


Fair, Isaac and Company, Incorporated
120 North Redwood Drive
San Rafael, CA 94903


         Re:      Registration Statement on Form S-3


Ladies and Gentlemen:

         I am General Counsel of Fair, Isaac and Company, Incorporated, a
Delaware corporation (the "Company"). In that capacity I have reviewed the
Registration Statement on Form S-3 to be filed by the Company with the
Securities and Exchange Commission under the Securities Act of 1933, as amended,
with respect to 210,508 shares of the Company's common stock, par value $0.01
per share (the "Common Stock"), to be offered and sold by certain stockholders
of the Company (the "Selling Stockholders"). As General Counsel, I am familiar
with the Company's Certificate of Incorporation and its By-Laws, as amended to
date. I have also examined such other documents, corporate records and
instruments as I have deemed necessary or appropriate for the purpose of this
opinion.

         Based upon the foregoing, I am of the opinion that such shares of
Common Stock of the Company to be offered and sold by the Selling Stockholders
have been duly authorized and legally issued and are fully paid and
nonassessable.

         I hereby consent to the filing of this opinion with the Securities and
Exchange Commission as Exhibits 5.1 and 23.2 and to the use of my name under the
caption "Legal Matters" in the Registration Statement and in the Prospectus
included therein.

                                        Very truly yours,

                                        /s/ Peter L. McCorkell

                                        Peter L. McCorkell
                                        Senior Vice President and
                                        General Counsel