In a study of 392 aortocoronary bypass (CABG) patients, we found 16 patients with a postoperative new Q wave, 29 patients with new intraventricular conduction disturbance, 17 patients with cardiogenic shock, and 14 patients with excessive CK-MB activity. Those criteria were considered as diagnostic of perioperative acute myocardial infarction (AMI). Listing the 392 patients in a Venn diagram: five patients had three positive criteria, eight had two, 43 had one, and 336 had none. Ventricular arrhythmia, supraventricular arrhythmia, or ST-T changes occurred in decreasing frequency in patients with a decreasing number of positive criteria. Five patients died postoperatively and in four a postmortem examination was available. Diagnostic criteria partly predicted autopsy findings. We conclude that the diagnostic criteria of perioperative myocardial infarction have a low diagnostic performance.