Recombinant human rt-PA was administered to 22 patients with deep vein thrombosis at a dosage of 30 to 120 mg/day (0.5 to 1.76 mg/kg body weight/24 hr) over 2 to 10 days. rt-PA induced phlebographically documented substantial recanalization in 18 of 21 patients. The lowest dose of 0.5 mg/kg/24 hr tested here was thrombolytically effective, whereas a dose of 0.95 mg/kg/24 hours and more led to hemorrhagic complications and premature discontinuation of therapy in four of six patients. Blood clotting analysis did not reveal any substantial decrease in fibrinogen concentrations, whereas the euglobulin clot lysis time and thromboelastography demonstrated a systemic fibrinolytic effect. Therapy with rt-PA can thus be considered as an alternative and effective method of therapy in treatment of deep vein thrombosis. The results of this study show that even a dosage of lower than 0.5 mg/kg/24 hr might prove to be effective. Further studies would be required to show whether the fibrin specificity of rt-PA leads to a superiority of this fibrinolytic substance over the conventional thrombolytic agents, streptokinase and urokinase.