A randomized study of the effects of intravenous streptokinase was performed in 214 patients with an acute myocardial infarction of less than 4 hours of whom 110 were included in the therapeutic group (SK) and 104 in the control group (C). Incidence of angiographic recanalization was higher in SK group (71 vs 28%, p less than 0.001) as that of non-significant residual coronary artery stenosis (less than 70%, 16% vs 3%, p less than 0.005), particularly in young patients (less than 45 years; 42% vs 8%, p less than 0.05). However, SK group presented a higher incidence of severe residual stenosis (90-99%) (SK, 42% vs C, 22%, p less than 0.01). Ejection fraction was higher among recanalized patients in both groups. Peak CPK-MB occurred earlier in SK group (13 vs 19 hours, p less than 0.001) and also among the recanalized patients of each group (SK, 12 vs 16 hours, p less than 0.001; C, 15 vs 21 hours, 0.002). The course of ST segment was similar in the 2 groups. The occurrence of ventricular arrhythmias within the first hour was greater in SK group (40% vs 20%, p less than 0.002), whereas the incidence of pericarditis (14% vs 35%, p less than 0.001) and of early mortality (less than 5 days, 2% vs 10%, p less than 0.02) was lower in SK group. The incidence of cardiac rupture, confirmed at necropsy in each of the 5 cases studied, was also lower in SK group (1 vs 8).(ABSTRACT TRUNCATED AT 250 WORDS)