Exhibit 5

                  [Letterhead of Cadwalader, Wickersham & Taft]


August 3, 2001

Dianon Systems, Inc.
200 Watson Boulevard
Stratford, Connecticut 06615

Re:      Registration Statement on Form S-4,
         relating to Common Stock par value $0.01 per share.
         ---------------------------------------------------


Ladies and Gentleman:

We have acted as special counsel for Dianon Systems, Inc., a Delaware
corporation (the "Company"), in connection with the preparation of the Company's
Registration Statement on Form S-4 (the "Registration Statement"), pursuant to
the Securities Act of 1933, as amended (the "Securities Act"), relating to the
registration by the Company of shares (the "Shares") of Common Stock, par value
$0.01 per share, of the Company to be issued in connection with the proposed
merger of UroCor, Inc., a Delaware corporation ("UroCor"), with the Company
pursuant to the terms of the Agreement and Plan of Merger, dated as of June 28,
2001, between the Company, UroCor Acquisition Corp. and UroCor (the "Merger
Agreement").

In rendering the opinions expressed below, we have examined and relied upon,
among other things, (i) the Merger Agreement, (ii) the Company's Certificate of
Incorporation, incorporated by reference as an exhibit to the Registration
Statement, (iii) the Company's By-laws, incorporated by reference as an exhibit
to the Registration Statement and (iv) originals or copies, certified or
otherwise identified to our satisfaction, of our such certificates, corporate,
public or other records, and other documents as we have deemed appropriate for
the purpose of rendering this opinion letter. In connection with such
examination, we have assumed the genuineness of all signatures, the authenticity
of all documents submitted to us as originals, the conformity to original
documents and instruments of all documents and instruments submitted to us as
copies or specimens, and the authenticity of the originals of such documents and
instruments submitted to us as copies or specimens. We have also made such
investigations of law as we have deemed appropriate.

In rendering this opinion we have assumed that prior to the issuance of any of
the Shares (i) the Registration Statement will have become effective under the
Securities Act, (ii) the common stockholders of UroCor will have approved and
adopted the Merger Agreement and the merger, (iii) the common stockholders of
the Company will have approved the issuance of shares of the




Company common stock as contemplated by the Merger Agreement, and (iv) the
transactions contemplated by the Merger Agreement are consummated in accordance
with the Merger Agreement.

On the basis of the foregoing, we are of the opinion that the Shares have been
duly authorized and the Shares, when issued and delivered in accordance with the
terms and conditions of the Merger Agreement, will be validly issued, fully paid
and non-assessable.

We are members of the Bar of the State of New York and the foregoing opinion is
limited to the laws of the State of New York, the federal laws of the United
States of America and the General Corporation Law of the State of Delaware.

We hereby consent to the filing of this opinion letter as an exhibit to the
Registration Statement and to the reference to Cadwalader, Wickersham & Taft in
the joint proxy statement/prospectus constituting a part of the Registration
Statement under the caption "Legal Matters," without admitting that we are
"experts" within the meaning of the Securities Act or the rules and regulations
of the Commission issued thereunder with respect to any part of the Registration
Statement, including this exhibit.

Yours very truly,


/s/ Cadwalader, Wickersham & Taft