Summary Plasma levels of thrombin-antithrombin III complexes (TAT) and prothrombin activation fragment F1+2(F1+2) were analysed in 80 patients with phlebographic proven acute deep vein thrombosis (DVT). TAT and F1+2 levels were measured at the time of diagnosis, at 6 and 24 h after the beginning of urokinase infusion, and at the end of fibrinolytic therapy. Patients were allocated into two subgroups according to their thrombus size: one group of 45 patients with a Marder score >25, and a second group of 35 patients with a score 75% (n=44) were compared with those cases where reperfusion was not achieved (lysis 25 and who had no other risk factors for thrombosis showed significant differences in the levels of TAT (p