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                                                                     Exhibit 5.1

                                 August 17, 1999

Board of Directors
Deltek Systems, Inc.
8280 Greensboro Drive
McLean, Virginia  22102

            Re:   Registration Statement on Form S-8

Gentlemen:

      We have acted as counsel to Deltek Systems, Inc., a Virginia corporation
(the "Company"), in connection with the preparation and filing by the Company of
its registration statement on Form S-8 (the "Registration Statement") under the
Securities Act of 1933, as amended, for the purpose of registering an aggregate
of 4,230 shares of Common Stock, $.001 par value (the "Common Stock"), issuable
upon exercise of options granted to Eric Brehm (the "Brehm Options").

      We have examined the Brehm Options, the Articles of Incorporation, as
amended, the By-Laws of the Company, the minutes of the various meetings and
consents of the Company's Board of Directors, originals or copies of such
records of the Company, agreements, certificates of public officials,
certificates of officers and representatives of the Company and others, and such
other documents, certificates, records, authorizations, proceedings, statutes
and judicial decisions as we have deemed necessary to form the basis of the
opinion expressed below. In such examination, we have assumed the genuineness of
all signatures, the authenticity of all documents submitted to us as originals
and the conformity to originals of all documents submitted to us as copies
thereof. As to various questions of fact material to such opinion, we have
relied upon statements and certificates of officers and representatives of the
Company and others.

      Based upon the foregoing, we are of the opinion that the 4,230 shares of
Common Stock issuable under the Brehm Options which are subject of the
Registration Statement have been duly authorized and, when issued and paid for
in accordance with the Brehm Options, will be duly authorized, fully paid and
nonassessable.

      We hereby consent to the use of this opinion as an exhibit to the
Registration Statement.

                                Yours very truly,

                                /s/ Hazel & Thomas, P.C.