From April 1968 to December 1972, 100 patients received emergency aortocoronary artery saphenous vein bypass grafts at the Cleveland Clinic Hospital. Thirty-seven of these 100 patients received grafts during acute myocardial infarction and 63 during impending infarction. Coronary arteriograms were obtained in all patients before operation. In the group with acute infarction, operations were performed within 12 hours after the onset of acute chest pain in 29 patients and from 3 to 14 days after infarction in 8. In the group with impending infarction, most patients received grafts within 3 to 4 hours after the onset of chest pain. Six of the 100 patients died in the postoperative period, 2 with acute and 4 with impending infarction. Thirty patients with acute infarction and 55 with impending infarction became asymptomatic. Sixteen of the 37 patients with impending infarction and 20 of the 63 patients with acute infarction were restudied. The graft patency rate in these patients was 92 percent. Coronary arteriography can be performed during acute and impending myocardial infarction with minimal risk. Aortocoronary saphenous vein bypass grafting can prevent the development of acute infarction when properly performed during the stage of impending infarction. It may also prevent the extension of acute infarction when , performed during the early phase of this lesion.