We performed a randomized trial comparing intracoronary administration of streptokinase versus dextrose placebo within six hours after the onset of symptoms of acute myocardial infarction in 40 patients. The base-line clinical, hemodynamic, and angiographic findings were similar in the control and streptokinase-treated groups. Reestablishment of flow occurred in 12 of 20 patients treated with streptokinase and in 2 of 20 given placebo (P less than 0.05). Left ventricular function, angiographic ejection fraction, and regional wall motion, measured before and immediately after intervention, and serial radionuclide ejection fractions, measured at treatment, at 12 days, and at 5 months, were compared according to type of treatment (streptokinase vs. placebo) and outcome of therapy (reperfusion vs. no reperfusion). No statistically significant differences between groups were found. Thus, although streptokinase was more effective than placebo in achieving reperfusion, we detected no improvement of left ventricular function as a result of reestablished coronary flow.