Exhibit 5.1

                   [CADWALADER, WICKERSHAM & TAFT LETTERHEAD]

                                   May 1, 2002

DIANON Systems, Inc.
200 Watson Boulevard
Stratford, CT  06615

            Re:   DIANON Systems, Inc.

Ladies and Gentlemen:

            We have acted as special counsel to DIANON Systems, Inc. (the
"Company") in connection with the preparation and filing by the Company with the
Securities and Exchange Commission of a Registration Statement on Form S-8 (the
"Registration Statement") under the Securities Act of 1933, as amended, dated as
of May 1, 2002, relating to the registration of up to 125,000 shares of the
Company's common stock, par value $.01 per share (the "Shares") to be issued
pursuant to the Non-ISO Stock Option Grant.

            In rendering the opinions set forth below, we have examined
originals, or copies certified or otherwise identified to our satisfaction, of
such documents corporate records and other instruments as we have deemed
necessary or appropriate.

            We express no opinion concerning the laws of any jurisdiction other
than the laws of the State of New York and the General Corporation Law of the
State of Delaware (including the statutory provisions, all applicable provisions
of the Delaware Constitution and reported judicial decisions interpreting the
foregoing). We are not licensed to practice law in the State of Delaware. Except
as described, we have neither examined no do we express any opinion with respect
to Delaware law.

            Based upon and subject to the foregoing, we are of the opinion that:

            1. The Shares have been duly authorized under the Company's
Certificate of Incorporation.

            2. The Shares, when issued and delivered by the Company pursuant to
the Non-ISO Stock Option Grant, will be validly issued, fully paid and
nonassessable.

            We are furnishing this opinion letter to you solely for your benefit
in connection with the registration referred to herein. We disclaim any
obligation to update this opinion letter for changes in fact or law, or
otherwise.

            We hereby consent to the filing of this opinion as an exhibit to the
Registration Statement. In giving this consent, we do not thereby concede that


we are within the category of persons whose consent is required under the
Securities Act of 1933, as amended, or the Rules and Regulations thereunder.

            Very truly yours,

            CADWALADER WICKERSHAM & TAFT


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