Treatment with streptokinase or heparin was allocated randomly to 20 patients with major pulmonary embolism verified by angiography. In addition, 4 patients treated with streptokinase and 1 patient treated with heparin were included in the trial prior to the start of treatment. Streptokinase of heparin was given for 72 hours and pulmonary angiography was repeated. The angiographic evidence of thrombolysis was significantly greater (p less than 0.01) in the 14 patients treated with streptokinase than in the 11 treated with heparin. In the heparin group, 1 patient died from massive embolism 15 hours after the start of treatment. In another patient who died 4 weeks later from cerebral glibolastoma, persistent massive embolism contributed to the fatal outcome. In the streptokinase group, 1 patient with a metastatic pulmonary carcinoma died 3 weeks after the start of treatment from gangrene of both legs following thrombotic occlusion of the inferior vena cava. Bleeding was more common after treatment with streptokinase than with heparin, but was not a serious problem in any patient. It is concluded that patients with life-threatening pulmonary embolism should be offered the benefits of streptokinase.