A study on coagulation-fibrinolysis system in surgical vascular disease was made to search for genesis of thrombosis and its therapy. Special consideration was taken into the availability of fibrinolytic enzyme for thrombosis. The results obtained in the study were as follows: 1) A marked increase of fibrinolytic activity was observed in most cases of aneurysm of the aorta, especially dissecting aneurysm, and antiplasmin level was low. 2) In arterial embolism, plasminogen activator was released from the site of a new thrombus into the blood. Then the activator spread throughout the entire body. The fibrinolytic activity reached the maximum level about 6 hours later and was reduced in a short time. 3) In phlebothrombosis, especially in the occlusion of the inferior vena cave, antiplasmin level was reduced. 4) Inmost cases of hemangioma, a marked increase of fibrinolytic activity was found in the blood drained from the affected area. Even though there would be no change of fibrinolytic activity in the systemic blood, laboratory data obtained from the local blood often showed those compatible with intravascular coagulation syndrome. 5) Postoperative use of hemostatics in vascular surgery had to be cautious, because of increased coagulation activity and decreased fibrinolysis activity that were induced by operation. Rather anticoagulation therapy might be necessary to prevent re-obstruction of the reconstructed vessels in occlusive vascular diseases. 6) Therefore, Urokinase would be better as a fibrinolytic agent in term of less side-effect and low antigenecity. 7) As arterial thrombus in the extremities was hardly dissolved with fibrinolytic agents and caused severer symptoms, reconstruction of an occluded artery should be performed as soon as possible and be followed by anticoagulant therapy. 8) Venous thrombus would be dissolved with fibrinolytic enzymes, if they were given within 3-4 days after the onset of symptoms. Hence it would be important to start with fibrinolytic therapy as soon as possible, rather than with surgical invention. Even if thrombus was not dissolved completely, collateral circulation would be improved enough to rehabilitate a patient without complaints. 9) Dosage of thrombolytic agents was determined from the following formula: Units of Urokinase to dissolve a clot at 6 hours × Estimated circulating blood volume=A daily dosage of Urokinase. Urokinase was administerd intravenously by continuous drip infusion with heparin and low molecular weight dextran. During the administration euglobulin lysis time was maintained at less than 60 minutes.