From September 1962 to May 1972 145 patients with acute or subacute deep vein thrombosis confirmed by phlebography were treated with streptokinase. During the same period 42 patients considered unfit for thrombolytic therapy were treated with herapin and oral anticoagulants. The results, assessed by repeat phlebography, in 93 of the patients treated with streptokinase were compared with those in 42 patients treated with heparin. The age, sex, and severity of occlusion were roughly similar in both groups. Streptokinase treatment was successful in 42 per cent, partially successful in 25 per cent, and unsuccessful in 32 per cent of the 93 patients compared with none, 10 per cent, and 88 percent respectively in the 42 patients treated with heparin. Streptokinase was more effective when the thrombus was in proximal rather than calf veins. Thrombi of more than six days old were readily lysed. Plasma fibrinogen levels were below 0-8 g/1 (80 mg/100 ml) in nearly all patients successfully treated. The incidence of pulmonary embolism was no greater with streptokinase than with heparin treatment. Only prolonged follow-up would show whether thrombolytic treatment would be effective in preventing late complications of deep vein thrombosis such as chronic venous insufficiency.