The authors have undertaken a prospective study on postoperative myocardial infarciton (PMI) following coronary artery bypass (CAB) grafting. PMI was diagnosed from electrocardiographic findings by the criteria of the American Heart Association. From Aug. 1, 1975 to Feb. 27, 1976, 198 patients (177 men, 21 women) underwent CAB. Their ages ranged from 31 to 71 years (mean, 49 yr). Of these patients, 18 (9%) sustained a PMI. A number of factors were analyzed to determine their influence on the occurrence rate of PMI. Preoperative factors wuch as the New York Heart Association classification, measurement of left ventricular function and evaluation of the severity of coronary artery disease were not helpful in predicting a predisposition to PMI. Endarterectomy was the only operative intervention associated with a higher inicidence of PMI (18%). Other factors such as the interval of cardiac anoxia and cardiopulmonary bypass and the number of vessels grafted did not seem to be associated with PMI. Low blood flow (less than 40 ml/min) through the graft could be correlated with the area of infarction in only 5 of 18 patients. Among the operated patients, postoperative ventricular arrhythmia was much more common in patients with PMI. The hospital mortality in this group was 11% (two patients).