Fourty-four patients with deep venous thrombosis of the leg diagnosed by phlebography and not more than five days old were treated with streptokinase. Oral anticoagulant therapy was started at the same time. Complete lysis of thrombi was obtained in 6 cases and partial one in 25 cases. Lysis of proximal thrombi was better than that of distal ones. The age of the thrombi had no effect on the lysis. Urticaria and anaphylactic reactions occurred in 8 patients and treatment was discontinued. Haemorrhagic complications were present in 8 cases but remission was obtained by dicontinuing streptokinase infusion. Commencing oral anticoagulant therapy simultaneously with streptokinase reduces duration of treatment without increasing the risks and the results of lysis are equally good as with earlier studies. The use of streptokinase is recommended in the treatment of proximal deep venous thrombosis. This treatment offers a possibility of preventing a postthrombotic syndrome. The frequency of various complications should however be borne in mind.