Exhibit 5.1

                  [CADWALADER, WICKERSHAM & TAFT LETTERHEAD]

                                  April 3, 2000



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 March 31, 2000, relating to the registration of up to 300,000 shares of the
Company's common stock, par value $.01 per share (the "Shares") to be issued
pursuant to the Company's Employee Stock Purchase Plan.

            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 federal laws of the United States of America, and the General
Corporation Law of the State of Delaware. While we are not licensed to practice
law in the State of Delaware, we have reviewed such applicable provisions of the
Delaware General Corporation Law as we have deemed appropriate in connection
with the opinions expressed herein.

            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. When issued and paid for as described in the Registration
Statement, the Shares 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.

                                    Very truly yours,

                                    CADWALADER WICKERSHAM AND TAFT




            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