The incidence of perioperative myocardial infarction determined by electrocardiogram was examined in 123 consecutive patients having only coronary artery bypass grafting for unstable angina pectoris, at Yale-New Haven Hospital from January 1974 to June 1975. The incidence of myocardial infarction and its mortality were correlated with clinical, haemodynamic, anatomical, and operative factors. Myocardial infarction occurred in 18% of all patients (22/123); 15 inferior, 6 anterior, and 1 anterolateral wall. Three factors appeared to be related to the occurrence of myocardial infarction: left main coronary artery disease (LMCD), (47%, 7/15), increased left ventricular end-diastolic pressure (LVEDP), (27%, 14/52), and cardiopulmonary bypass time more than 60 minutes (24%, 21/88). The mortality of perioperative myocardial infarcation was 13.6% (3/22), while for patients without perioperative myocardial infarction the mortality was 2% (2/101). The overall operative mortality was 4% (5/123). The risk of perioperative myocardial infarction is significantly increased by left main coronary artery disease, increased left ventricular end-diastolic pressure, and cardiopulmonary bypass time more than 60 minutes, in patients undergoing coronary artery surgery for unstable angina pectoris. The mortality of perioperative myocardial infarction is high (13.6%) in patients with unstable angina.