Randomly assigned streptokinase or heparin therapy was studied in 50 patients with deep vein thrombosis of less than 2 weeks' duration. Venography was performed prior to therapy, after 3 days, and after 10 days. Two radiologists who were unaware of the patient's therapy compiled a single average lytic score for each patient at each time interval. Lysis of venous thrombi was significantly greater with streptokinase than with heparin after 3 days, but not after 10 days of treatment. Lytic scores achieved with streptokinase were significantly better than those achieved with heparin therapy (P less than .01) in male patients who were symptomatic for 3 days or less. In females, regardless of the duration of symptoms, thrombolytic results obtained with streptokinase were not significantly different than results obtained with heparin.