To evaluate the efficacy of LDH isoenzymes in the detection of myocardial infarction in patients undergoing coronary bypass surgery, 73 patients were studied pre- and postoperatively by ECG, CPK, SGOT, total serum LDH, and LDH isoenzyme measurements. A reversal of the LDH1:LDH2 ratio was considered indicative of myocardial necrosis. Accordingly, the patients were separated into two groups: Group A (23 patients) who demonstrated an LDH1:LDH2 ratio exceeding 1.0 and Group B (50 patients) who failed to reveal an LDH1;LDH2 reversal. The two groups were similar in regard to preoperative evaluation and operative procedure performed. The postoperative ECG findings were significantly different. In Group A 18 of 23 patients (78 per cent) developed significant new Q waves. This occurred in only one patient in Group B. Significant arrhythmias occurred in 70 per cent of the patients in Group A as compared to 14 per cent of those patients in Group B. Severe congestive heart failure and/or clinical evidence of shock occurred in 39 per cent of Group A patients and in none in Group B. The results of our study indicate that the reversal of the LDH1:LDH2 ratio is a valuable tool for the evaluation of postoperative myocardial infarction.