<PAGE>

                                                                     EXHIBIT 5.1

                            THOMPSON & KNIGHT L.L.P.

                            ATTORNEYS AND COUNSELORS

                         1700 PACIFIC AVENUE, SUITE 3300
                            DALLAS, TEXAS 75201-4693
                                 (214) 969-1700
                               FAX (214) 969-1751


                               September 21, 2000



Lone Star Technologies, Inc.
15660 North Dallas Parkway, Suite 500
Dallas, Texas   75248

         Re:      LONE STAR TECHNOLOGIES, INC. 1985 LONG-TERM INCENTIVE PLAN

Ladies and Gentlemen:

         We have acted as counsel for Lone Star Technologies, Inc., a Delaware
corporation (the "Company"), in connection with the registration under the
Securities Act of 1933, as amended (the "Act"), of an aggregate of 1,000,000
shares (the "Shares") of the common stock, $1.00 par value per share (the
"Common Stock"), of the Company issuable pursuant to its 1985 Long-Term
Incentive Plan (the "Plan"). A registration statement on Form S-8 (the
"Registration Statement") covering the issuance and sale of the Shares from time
to time under the Plan has this date been filed under the Act with the
Securities and Exchange Commission (the "Commission").

         In reaching the conclusions expressed in this opinion, we have
examined executed copies of the Registration Statement and all exhibits
thereto. We have also examined and relied upon the original, or copies
certified to our satisfaction, of (i) the Certificate of Incorporation and
the Bylaws of the Company, (ii) minutes and records of the corporate
proceedings of the Company with respect to the issuance of the Shares and
related matters, (iii) the Plan and (iv) such other agreements and
instruments relating to the Company as we have deemed necessary or
appropriate for purposes of the opinions hereinafter expressed. In rendering
such opinions, we have made such further investigation and inquiries relevant
to the transactions contemplated by the Registration Statement as we have
deemed necessary as a basis for the opinions expressed herein and have
relied, to the extent we deemed reasonable, on certificates and certain other
information provided to us by officers of the Company and public officials as
to matters of fact of which the maker of each such certificate or the person
providing such other information had knowledge. Furthermore, in rendering
such opinions, we have assumed that the signatures on all documents examined
by us are genuine, that all documents and corporate record books submitted to
us as originals are accurate and complete, and that all documents submitted
to us as copies are true, correct and complete copies of the originals
thereof.

         In issuing the opinions hereinafter expressed, we do not purport to be
experts in the laws of any jurisdiction other than the State of Texas and the
United States of America.

<PAGE>

Lone Star Technologies, Inc.
September 21, 2000
Page 2



         Based solely upon the foregoing, and limited in all respects as stated
above, we are of the opinion that the Shares registered pursuant to the
Registration Statement have been duly and validly authorized by the Company, and
when issued, sold and delivered in the manner and for the consideration
described in the Plan, will be validly issued, fully paid and nonassessable.

         We hereby consent to the filing of this opinion as an exhibit to the
Registration Statement. In granting this consent, we do not thereby admit that
we come within the category of persons whose consent is required pursuant to
Section 7 of the Act or the rules or regulations of the Commission thereunder.

                                                   Respectfully submitted,

                                                   /s/ Thompson & Knight L.L.P.