Urokinase (average 1670 IU/kg X h) and heparin (average 17 IU/kg X h) was infused over several days into the main pulmonary artery of 12 patients with acute massive pulmonary embolism, after diagnostic pulmonary angiography. Pulmonary artery pressures and blood clotting values were serially recorded. There was convincing improvement in the clinical condition of all patients: none died or had a recurrence of embolism. The heart rate and pulmonary vascular obstruction decreased, mean pulmonary artery pressure fell and arterial pO2 rose. There were other, also statistically significant, changes in a decrease of fibrinogen, rise in thrombin time and decrease in haemoglobin concentration. Pulmonary artery pressure after 24 hours had fallen slightly, after 5.8 days significantly. In two patients a haemoglobin fall of more than 3 g/dl required blood transfusion. The findings indicate that after massive pulmonary embolism local continuous infusion of urokinase can, after 5-6 days, normalize pulmonary haemodynamics; the pulmonary angiogram shows almost complete disappearance of the pulmonary vascular obstruction.