Thirteen of 199 consecutive patients undergoing coronary artery bypass surgery revealed definite perioperative myocardial infarction (PMI) in electrocardiography (ECG). The occurrence of PMI was not higher in the group of 44 patients who had intermittent aortic cross-clamping than in those patients treated with cold chemical cardioplegia. In 83 patients serum MB isoenzyme of creatine kinase (CK) and LD1 Isoenzyme of lactic dehydrogenase were determined. Patients with unchanged ECG had peak CK-MB levels of 0 to 49 U/l (mean 18.7 U/l ± 2.6 SEM) at 18 hours postoperatively while patients with PMI showed CK-MB levels of 64-350 U/l (mean 207 ± 53 U/l); the difference was significant (p < 0.01). In patients with unchanged ECG, LD1, was 139 ± 19 U/l and 594 ± 95 U/l in those with PMl (p < 0.01). Risk factors for PMI were: age ≥ 60 years, coronary endarterectomies, or cardiopulmonary bypass time ≥ 100 minutes. One patient died of PMI while the remaining patients had postoperative courses comparable to those patients without PMI.