Exhibit 5.1

KIRKLAND & ELLIS LLP

AND AFFILIATED PARTNERSHIPS

Citigroup Center

153 East 53rd Street

New York, New York 10022-4611

212 446-4800 www.kirkland.com Facsimile: 212 446-4900

March 15, 2004

Cable Design Technologies Corporation

1901 North Roselle Road

Schaumburg, IL 60195

Re: Cable Design Technologies Corporation—Registration Statement on Form S-3

Ladies and Gentlemen:

We are acting as special counsel to Cable Design Technologies Corporation, a Delaware corporation (the “Company”), in connection with its registration statement on Form S-3 (the “Registration Statement”), initially filed with the Securities and Exchange Commission on December 5, 2003 (the “Commission”), relating to resales of up to $110,000,000 aggregate principal amount of the Company’s 4% Convertible Subordinated Debentures due 2023 (the “Debentures”) and 12,175,549 shares (as such number may be adjusted between the Company and U.S. Bank, National Association, as trustee (“the Trustee”)) of Common Stock, par value $0.01 per share, of the Company (the “Common Stock”) issuable upon conversion of the Debentures in accordance with the Indenture incorporated by reference as Exhibit 4.3 to the Registration Statement (the “Indenture”). This opinion letter is furnished to you at your request to enable you to fulfill the requirements of Item 601(b)(5) of Regulation S-K in connection with the Registration Statement.

For purposes of this opinion letter, we have examined copies of the following documents:

1. An executed copy of the Registration Statement.

2. An executed copy of the Indenture.

3. An executed copy of the Global Debenture representing the Debentures, dated as of July 8, 2003.

4. The Amended and Restated Certificate of Incorporation of the Company, as certified by the Secretary of the State of the State of Delaware on July 2, 2003.

5. The Bylaws of the Company.

6. Resolutions of the Board of Directors of the Company adopted at a meeting of the Board of Directors of the Company on June 27, 2003, relating to the offering of the Debentures and arrangements in connection therewith.

7. Copies of all certificates and other documents delivered at the closing of the purchase and sale of the Debentures.

For purposes of this opinion letter, we have assumed the authenticity of all documents submitted to us as originals, the conformity to the originals of all documents submitted to us as copies and the authenticity of the originals of all documents submitted to us as copies. We have also assumed the legal capacity of all natural persons, the genuineness of the signatures of persons signing all documents in connection with which this opinion is rendered, the authority of such persons signing on behalf of the parties thereto other than the Company and the due authorization, execution and delivery of all documents by the parties thereto other than the Company. As to any facts material to the opinions expressed herein, we have relied upon the statements and representations of officers and other representations of the Company and others.

Chicago London Los Angeles San Francisco Washington, D.C.

Our advice on every legal issue addressed in this letter is based exclusively on the internal laws of the State of New York, the General Corporation Law of the State of Delaware and the federal law of the United States of America, and represents our opinion as to how that issue would be resolved were it to be considered by a court in the jurisdiction which enacted such law.

Based upon and subject to the foregoing qualifications, assumptions and limitations and the further limitations set forth below, we hereby advise you that in our opinion:

1. The Debentures constitute valid and binding obligations of the Company, enforceable against the Company in accordance with their terms.

2. The shares of Common Stock issuable upon conversion of the Debentures, when issued in accordance with the terms of the Debentures, will be validly issued, fully paid and non-assessable.

Our opinion in paragraph 1 of this opinion letter is subject to: (i) the effect of bankruptcy, insolvency, fraudulent conveyance and other similar laws and judicially developed doctrines in this area such as substantive consolidation and equitable subordination; (ii) the effect of general principles of equity; and (iii) other commonly recognized statutory and judicial constraints on enforceability including statutes of limitations. “General principles of equity” include but are not limited to: principles which limit the availability of specific performance and injunctive relief; principles which limit the availability of a remedy under certain circumstances where another remedy has been elected; principles requiring reasonableness, good faith and fair dealing in the performance and enforcement of an agreement by the party seeking enforcement; principles which may permit a party to cure a material failure to perform its obligations; and principles affording equitable defenses such as waiver, laches and estoppel.

We do not find it necessary for the purposes of this opinion, and accordingly we do not purport to cover herein, the application of the securities of “Blue Sky” laws of the various states to the Debentures and the Common Stock issuable upon conversion of the Debentures.

This opinion letter is limited to the specific issues addressed herein, and no opinion may be inferred or implied beyond that expressly stated herein. We assume no obligation to revise or supplement this opinion should the present laws of the State of New York, the General Corporation Law of the State of Delaware or the federal law of the United States be changed by legislative action, judicial decision or otherwise.

We hereby consent to the filing of this opinion letter with the Commission as Exhibit 5.1 to the Registration Statement. We also consent to the reference to our firm under the heading “Legal Matters” in the Registration Statement. In giving this consent, we do not thereby admit that we are in the category of persons whose consent is required under Section 7 of the Securities Act of 1933 or the rules and regulations of the Commission.

Very Truly Yours,

/S/ KIRKLAND & ELLIS LLP

KIRKLAND & ELLIS LLP