To test the efficacy of thrombolytic therapy in massive pulmonary embolism, we conducted a prospective randomized controlled trial. Eight patients were randomized to receive either 1,500,000 IU of streptokinase in 1 hour through a peripheral vein followed by heparin or heparin alone. All patients had major risk factors for deep vein thrombosis (DVT) and were considered to have high clinical suspicion for pulmonary embolism (PE). At baseline all patients had a similar degree of systemic arterial hypotension, pulmonary arterial hypertension, and right ventricular dysfunction. The time of onset of cardiogenic shock in both groups was comparable (2.25 ±0.5 hours in the streptokinase group and 1.75 ±0.96 hours in the heparin group). The four patients who were randomized to streptokinase improved in the first hour after treatment, survived, and in 2 years of follow-up are without pulmonary arterial hypertension. All four patients treated with heparin alone died from 1 to 3 hours after arrival at the emergency room (p=0.02). Post-thrombolytic therapy the diagnosis of PE was sustained in the streptokinase group by high probability V/Q lung scans and proven DVT. A necropsy study performed in three patients in the heparin group showed massive pulmonary embolism and right ventricular myocardial infarction, without significant coronary arterial obstruction. The results indicate that thrombolytic therapy reduces the mortality rate of massive acute pulmonary embolism.