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                                                                     EXHIBIT 5.1

          OPINION OF COUNSEL REGARDING LEGALITY AND CONSENT OF COUNSEL

                         {Letterhead of Ross & Hardies}

                               September 15, 2000

SPSS Inc.
233 South Wacker Drive
Chicago, Illinois  60601

                Re:  Registration Statement on Form S-8

Ladies and Gentlemen:

                You have requested our opinion with respect to the registration
by SPSS Inc. ("SPSS" or the "Company") pursuant to a Registration Statement on
Form S-8 (the "Registration Statement") under the Securities Act of 1933, as
amended (the "Act"), of an aggregate of 626,000 shares of the Company's Common
Stock, $.01 par value per share (the "Common Stock"), issuable upon the exercise
of options (the "Options") to purchase Common Stock as issued pursuant to the
SPSS 1999 Employee Equity Incentive Plan (the "Plan").

                In so acting, we have examined originals or copies, certified or
otherwise identified to our satisfaction, of such documents, corporate records,
certificates of public officials and other instruments and have conducted such
other investigations of fact and law as we have deemed relevant and necessary to
form a basis for the opinions hereinafter expressed. In conducting such
examination, we have assumed (i) that all signatures are genuine, (ii) that all
documents and instruments submitted to us as copies conform with the originals,
and (iii) the due execution and delivery of all documents where due execution
and delivery are a prerequisite to the effectiveness thereof. As to any facts
material to this opinion, we have relied upon statements and representations of
officers and other representatives of the Company and certificates of public
officials and have not independently verified such facts, but have no actual
knowledge of the inaccuracy or incompleteness of the same.

                Based upon the foregoing, it is our opinion that the Common
Stock issuable upon the proper exercise of Options granted pursuant to the Plan
will be validly issued, fully paid and non-assessable when issued in accordance
with the Plan.

                We express no opinion as to the laws of any jurisdiction other
than the State of Illinois, the United States of America, and, solely with
respect to matters of corporate organization and authority, the General
Corporation Law of the State of Delaware. We are not admitted to the practice of
law in the State of Delaware. Insofar as the foregoing opinion relates to
matters that would be controlled by the substantive laws of any jurisdiction
other than the United States of America, the General Corporation Law of the
State of Delaware, with respect to matters of corporate organization and
authority, or the State of Illinois, we have assumed that the substantive laws
of such jurisdiction conform in all respects to the internal laws of the State
of Illinois.

                We hereby consent to the reference to our firm in the
Registration Statement relating to the registration of the 626,000 Shares of
Common Stock issuable upon exercise of the Options described above.

                                             Very truly yours,


                                             /s/  Ross & Hardies
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                                             ROSS & HARDIES