Coagulation and fibrinolysis were studied in normal subjects and patients with Buerger's disease (TAO), arteriosclerosis obliterans (ASO), acute arterial thrombosis (AAT) and acute deep venous thrombosis (ADVT); the mean ages were 32.8, 42, 5, 63.5, 64.8 and 60.2 years respectively. β-TG significantly elevated in all the patient groups compared with the normal subjects. FPA, FPBβ 15-42 and D-dimer significantly elevated in ASO, AAT and ADVT groups compared with the normal subjects and TAO groups. Therefore in ASO, AAT and ADVT groups, there might be platelet activation, hypercoagulability and increased fibrinolysis, whereas TAO group seemed to have only platelet activation. These results might be dependent on difference in the amount of unorganized thrombi among the patient groups. Low activity of α2-PI showed effective fibrinolysis in thrombolytic therapy (urokinase and t-PA) by means of ultiple regression analysis. Using fibrin plate method, fibrinolysis area appeared when α2-PI was less than 50% in urokinase therapy. Judging from the measurement of α2-PI after administration of urokinase, it was expected that α2-PI was almost maintained below 50% when 600, 000 units of urokinase were intravenously infused for 3 hours on the first day and an intravenous administration of 300, 000 units of urokinase for 1.5 hours were given daily from the following day.