The serial measurement of postoperative serum enzymes has been proposed as an important method for identifying myocardial infarction following aortocoronary graft surgery. Serum glutamic oxalacetic transaminase, creatine phosphokinase and lactic dehydrogenase levels were determined during the initial five days following direct coronary artery surgery in 112 patients. Enzyme test results were analyzed by frequency distribution plots. Twelve patients (10.7%) developed definite electrocardiographic evidence of myocardial infarction within two weeks of surgery, seven on the first postoperative day. A general correlation of higher serum enzyme values and electrocardiographic evidence for myocardial infarction was established. However, the use of arbitrarily selected 90th percentile enzyme levels yielded a substantial number of false-negative and false-positive results as compared with electrocardiographic diagnosis. The 90th percentile levels were substantially higher following multi-vessel surgery, compared with single-vessel surgery. Thus, analysis of serum enzymes following coronary surgery was found to be a useful, but not definitive adjunct in identifying patients suspect for operative infarction.