To evaluate the incidence of perioperative myocardial infarction (PMI), serial determinations of serum creatine kinase isoenzymes (CK-MB), electrocardiograms (ECGs), and pyrophosphate myocardial scans were performed in 112 patients undergoing isolated coronary bypass grafting. An abnormal increase in total CK-MB liberation (Q greater than 9.8 IU ml-1 kg) occurred in 25 patients (22.3%), new Q waves were present at ECG in 10 patients (8.9%), and the pyrophosphate myocardial scan was abnormal in 13 patients (11.6%). All tests were negative in 81 patients (72.3%). A diagnosis of PMI was established if confirmed by at least two of the techniques; this diagnosis was made in 15 patients (13.4%). The pattern of CK-MB liberation in patients with a PMI, characterized by a high peak and a prolonged release, was significantly different from that of patients without a PMI. The most important predictive factor for PMI was the duration of myocardial ischemia during the operation. Patients who had a PMI had more frequent early complications, and their prognosis at 2 years showed a 51% probability of remaining free of new cardiac events as compared to 96% for the group of patients without a PMI (p less than 0.001). PMI is not a benign complication of coronary bypass, and its detection appears improved by a combination of diagnostic tests.