The incidence of myocardial infarction associated with aortocoronary bypass is difficult to determine because of widespread disagreement in evaluating the criteria for its detection. A prospective study consisting of 100 consecutive patients undergoing surgery for aortocoronary bypass was initiated. All patients were evaluated before and after surgery with electrocardiograms, heart scans of the infarct, and determinations of the level of the MB isoenzyme of creatine phosphokinase. A figure determined to be the upper limit of the expected rise in the level of the isoenzyme resulting from cardiac manipulation was selected, and 20 percent of the patients had more than the expected rise in the enzymatic level. Approximately one-half of these patients had abnormal ECGs or scans (or both) as well. The determination of the level of isoenzyme is very sensitive, and a normal level would exclude infarction. Elevations of concentrations of the enzyme due to minor myocardial cellular alterations are frequent. For this reason, in addition to an elevation of the concentration of the enzyme above the expected rise, the presence of an abnormal scan or ECG (or both) is necessary to make the diagnosis of perioperative infarction.