Urokinase, an enzymatic fibrinolytic activator, was used as the principal form of therapy in the treatment of ten patients with acute pulmonary thromboembolism. Although an absolute statement cannot be made about the efficacy of the drug in the management of this disease, this study seems to demonstrate that lysis of thromboemboli is considerably hastened by the use of this drug. Rapid lysis of emboli is necessary for the successful management of acute massive pulmonary thromboembolism. Three patients in this series previously could have been considered candidates for pulmonary embolectomy. All survived following urokinase therapy. After this experience, we have modified our surgical indications for the treatment of pulmonary thromboembolism to consider for urokinase therapy all patients with massive bilateral pulmonary emboli in shock and those in whom inferior caval ligation is usually recommended for the prevention of further embolization.