EXHIBIT 5.1

                               September 30, 1999

Predictive Systems, Inc.

145 Hudson Street

New York, New York 10013

           Re:  Predictive Systems, Inc.--Registration Statement on

                Form S-1 (File No. 333-84045)

Ladies and Gentlemen:

    We have acted as counsel to Predictive Systems, Inc., a Delaware corporation
(the "Company"), in connection with the proposed issuance and sale by the
Company of up to 4,600,000 shares of the Company's Common Stock (the "Shares")
pursuant to the Company's Registration Statement on Form S-1 (the "Registration
Statement") filed with the Securities and Exchange Commission under the
Securities Act of 1933, as amended (the "Act").

    This opinion is being furnished in accordance with the requirements of Item
16(a) of Form S-1.

    We have reviewed the Company's charter documents and the corporate
proceedings taken by the Company in connection with the issuance and sale of the
Shares. Based on such review, we are of the opinion that the Shares have been
duly authorized, and if, as and when issued in accordance with the Registration
Statement and the related prospectus (as amended and supplemented through the
date of issuance) will be legally issued, fully paid and nonassessable.

    We consent to the filing of this opinion letter as Exhibit 5.1 to the
Registration Statement and to the reference to this firm under the caption
"Legal Matters" in the prospectus which is part of the Registration Statement.
In giving this consent, we do not thereby admit that we are within the category
of persons whose consent is required under Section 7 of the Act or the rules and
regulations of the Securities and Exchange Commission promulgated thereunder.

    This opinion letter is rendered as of the date first written above and we
disclaim any obligation to advise you of facts, circumstances, events or
developments which may be brought to our attention through the time of
effectiveness of the Registration Statement, and which may alter, affect or
modify the opinion expressed herein. Our opinion is expressly limited to the
matters set forth above and we render no opinion, whether by implication or
otherwise, as to any other matters relating to the Company or the Shares.

                                          Very truly your,

                                          /s/ Brobeck, Phleger & Harrison LLP

                                          BROBECK, PHLEGER & HARRISON LLP