To clarify the value of isoenzymes in the detection of acute myocardial injury (AMI) associated with coronary artery bypass grafting, 350 consecutive patients (804 grafts) were evaluated with serial electrocardiograms (ECG) and serum levels of serum glutamic oxaloacetic transaminase (SGOT), total creatinine phosphokinase (CPK), and lactic dehydrogenase (LDH) on Days 0, 1,2,3,4,5, and 7. Graft patency 1 to 6 months postoperatively was 92%. Sixty patients (18%) had ECG evidence of AMI. Four patients died (1.1%) Of the 185 patients with serial LDH isoenzyme analysis, 46 had elevated cardiac fraction (LDH-1). Only 35 of these had AMI by ECG. The others had nonspecific ECG changes, but associated enzymatic andhemodynamic evidence of AMI. The CPK-MB band was elevated in all patients in the immediate postoperative period, but did not correlate with ECG, hemodynamic, or enzymatic evidence of AMI. The diagnosis of AMI following coronary artery bypass could not be substantiated by evaluation of total LDH, CPK, or CPK-MB. LDH-1 is specific in detecting AMI after a coronary artery bypass graft.