30 patients with deep vein thrombosis were treated with a combination of urokinase and heparin. Clinically relevant improvement was achieved in 2/3 of them with appr. 40,000 IU/h (1,000,000 IU/d) urokinase administered over a period of several days. This indicates that urokinase at this dosage offers a valuable alternative or supplementation to fibrinolytic therapy with streptokinase. With the dosage employed, routine blood coagulation tests are only minimally affected, although a strong enhancement of fibrinolytic activity can be demonstrated by the euglobulin clot lysis time. Plasminogen depletion - as is usually observed with streptokinase therapy - does not occur. Urokinase is well tolerated and there is only a very moderate bleeding tendency. The cost per day of urokinase therapy at the dosage employed is approximately twice that of customary streptokinase therapy.