Exhibit 5.1


                     [Letterhead of Pillsbury Winthrop LLP]


                                January 29, 2003

Fair, Isaac and Company, Incorporated
200 Smith Ranch Road
San Rafael, CA  94903-1996

      Re:   Registration Statement on Form S-8

Ladies and Gentlemen:

      We are acting as counsel for Fair, Isaac and Company, Incorporated, a
Delaware corporation (the "Company"), in connection with the registration under
the Securities Act of 1933, as amended (the "Securities Act"), of: (a) 2,932,093
shares of common stock, par value $0.01 per share, of the Company (the "Common
Stock"), issuable pursuant to the Company's 1992 Long-Term Incentive Plan (the
"Plan"); and (b) the Company's Preferred Stock Purchase Rights that will be
attached to and represented by the certificates issued for shares of the Common
Stock (which Preferred Stock Purchase Rights have no market value independent of
the Common Stock, to which they are attached).

      We are of the opinion that such shares of Common Stock, and the Company's
Preferred Stock Purchase Rights, have been duly authorized and, when issued in
accordance with the Plan, will be legally issued, fully paid and non-assessable,
subject to bankruptcy, insolvency, reorganization, moratorium or other similar
laws affecting or relating to the rights of creditors generally, and to general
principals of equity. The foregoing opinion is limited to the federal laws of
the United States, the laws of the State of California, and the General
Corporation Law of the State of Delaware, and we are expressing no opinion as to
the effect of the laws of any other jurisdiction.

      We hereby consent to the filing of this opinion with the Securities and
Exchange Commission as Exhibit 5.1 to the Company's Registration Statement on
Form S-8.

                                    Very truly yours,

                                    /s/ PILLSBURY WINTHROP LLP