Clinical course of the disease and the formation of the necrotic focus (as evidenced by precordial mapping from 35 ECG leads) were assessed in 88 patients with acute myocardial infarction, treated with 1,000,000 IU urokinase, in comparison to 41 untreated control patients. Thrombolytic therapy, started within the first 6 hours of myocardial infarction, was associated with a better clinical course of the disease. Urokinase administration after 6 hours from the onset of the symptoms produced no clinical improvement, did not limit the necrotic focus and failed to reduce mortality, as compared to the controls. The assessment of the efficiency of thrombolytic treatment demonstrated an at least 30% increment of venous flow in 45% of patients. Intravenous urokinase administration was accompanied by an activation of blood fibrinolytic system, and there was a tendency to fibrinolysis depression on the day following the administration.