A controlled, prospective study comparing streptokinase and heparin treatment has been completed in 51 patients presenting with acute proximal venous thrombosis of less than 8 days' clinical duration. Patients were studied by means of pre-treatment, post-treatment, 3- and 12-monthly phlebography and pulmonary perfusion scanning and were followed up at 3-monthly intervals. Of the 26 patients randomized to receive streptokinase, therapy was stopped in 3 because of complications. Phlebography 5 days after starting treatment showed 80--100 per cent lysis in 17 of the 23 patients who completed the course of streptokinase. Two patients later developed partial rethrombosis. One patient developed an asymptomatic pulmonary embolus during treatment. During follow-up (mean 19 months) only 1 of the 17 patients with 80--100 per cent lysis developed postphlebitic symptoms, 3 patients died of unrelated causes and 1 patient was lost to follow-up. In patients randomized to heparin therapy no significant lysis was achieved in any of the 25 patients and only 2 of these patients were found to have asymptomatic legs on follow-up. Two patients in this group died and autopsy confirmed massive pulmonary embolus during treatment. These data suggest that streptokinase is superior to heparin in the treatment of acute proximal venous thrombosis of less than 1 week's clinical duration especially if the thrombus is largely non-occlusive. It must be stressed that in order to avoid the bleeding complications of thrombolytic therapy, streptokinase must not be used within 10 days of major surgery, or even longer after vascular, neurosurgical or eye operations.