The incidence of perioperative myocardial infarction (MI) determined by electrocardiogram was examined in 168 consecutive patients having only coronary artery bypass grafting at Yale-New Haven Hospital from July 1974 to June 1975. The incidence of MI and its mortality were correlated with clinical, hemodynamic, anatomic, and operative factors. Perioperative MI occurred in 23% of all patients (39/168); 26 in the inferior, 11 in the anterior, and two in the anterolateral wall. Three factors appeared related to the occurrence of MI: 1) abrupt propranolol withdrawal 24 hours prior to surgery (Prop) (32%, 33/103); 2) left main coronary artery disease (LMCD) (37%, 7/19); and 3) cardiopulmonary bypass longer than 60 minutes (CPB) (23%, 30/128). To more precisely predict MI, combinations of factors were examined. The combination of LMCD and CPB was 39%, (7/18) while the absence of either yielded an incidence of only 5.1% (2/39) (P less than 0.001). The mortality of patients with MI was 15.4% (6/39) while in patients without MI the mortality was 1.6% (2/129). We conclude that the risk of perioperative MI is significantly increased by abrupt propranolol withdrawal 24 hours before surgery, left main coronary artery disease, and cardiopulmonary bypass longer than 60 minutes in patients undergoing coronary artery bypass grafting. The mortality of perioperative MI is high, despite previous reports of the benignity of perioperative myocardial infarction.