The efficacy of intracoronary administration of urokinase was evaluated in 514 patients with acute myocardial infarction (anterior, 296 patients; inferior, 195; lateral or posterior, 18; and anterior and inferior, five). The time between onset of chest pain and coronary arteriography was 0.5 to 81.0 hr with an average of 5.0 hr. Initial administration of nitrates resulted in recanalization of the coronaries in 9.3%. Subsequently, urokinase was infused into the coronary arteries, and coronary thrombolysis was successfully achieved in 66.8%. The success rate was low in a group with average infusion speed of more than 30,000 units/min or with a total dose of urokinase of 480,000 units or less. Complications, mainly arrhythmias, were present in 111 patients (33.2%) of the 334 who had successful thrombolysis and in 18 patients (10.8%) of the 166 with unsuccessful thrombolysis, but serious hemorrhage was rare and no fatal case was reported. Patients who had successful thrombolysis had less in-hospital mortality than those who did not (6.3 vs 13.3%). Thus, coronary thrombolysis can be achieved effectively and relatively safely with a sufficient amount of intracoronary urokinase administration in acute myocardial infarction.