In 18 cases with primary subclavian-axillary vein thrombosis fibrinolytic therapy was performed with urokinase in combination with heparin. The thrombolytic efficacy clearly depended on the thrombus age and the dose of urokinase applied. Under treatment with a median initial maintenance dosage of urokinase of 1,000-2,000 IU/kg/h (loading dose 150,000-250,000 IU urokinase) in combination with heparin (15-17 U kg/h) in mine of 11 patients (82%) with recently developed (8 days or less) thrombosis, a nearly complete deobliteration of the venous system was observed. In the case with thrombosis of more than 10 days no alteration of the venous occlusions could be seen. Relevant side effects did not occur. Our results emphasize urokinase therapy of acute subclavian-axillary vein thrombosis and permit general inferences concerning the efficacy and the dosage requirements of the thrombolytic substance urokinase.