Among 263 patients with acute myocardial infarction, 141 were treated with urokinase (UK group) and 122 received no urokinase (conventional group). Urokinase (UK) was administered intracoronarily in 55 cases; intravenously (mainly 1,920,000 units) in 64 cases; and intravenously and intracoronarily in 22 cases. The mortality rates were ascertained three months after admission and during the mean follow-up periods of 17.5 months for the UK group and 24.5 months for the conventional group. The three month mortality was significantly lower in the UK group (10.6%, 15 cases) than in the conventional group (23.8%, 29 cases) (p less than 0.01). The mortality during the entire follow-up period was also lower in the UK group (14.2%, 20 cases) than in the conventional group (26.2%, 32 cases) (p less than 0.05). Fatalities due to cardiac rupture, ventricular fibrillation, cardiac failure, cardiogenic shock and recurrent infarction were uniformly less in the UK group. It was concluded that coronary thrombolysis is an effective means of reducing mortality in acute myocardial infarction.