Deep vein thrombosis may begin during surgery with the tourniquet inflated. Arterial levels of fibrinopeptide A, thrombin-antithrombin complexes, D-dimer, tissue plasminogen activator (t-PA) activity, and t-PA antigen were measured before surgery, during surgery with the tourniquet inflated, and following deflation of the tourniquet in 12 patients undergoing total knee arthroplasty. Minimal increases in fibrinopeptide A, thrombin-antithrombin complexes, and D-dimer were noted during surgery with the tourniquet inflated, but significant increases occurred immediately following deflation of the tourniquet. In 10 patients, intravenous heparin administration significantly suppressed the rise in fibrinopeptide A, but did not significantly alter the increases in either thrombin-antithrombin complexes, D-dimer, t-PA antigen, or t-PA activity. This study provides further evidence that deep vein thrombosis begins during surgery.