The diagnostic accuracy of the 12-lead scalar electrocardiogram (ECG) for the presence or absence of acute transmural myocardium infarction (MI) was studied in 25 patients who died while in hospital for treatment of ischemic chest pain. Detailed autopsy studies revealed myocardial necrosis in 17 patients, with the ECG accurate in 11 (44%) of the 25 patients. None of the five patients with autopsy-proven subendocardial infarction had abnormal Q waves. The admission ECG was diagnostic in only 4 of the 17 patients (24%) with infarction. The creatine kinase (CK) and CK-MB results agreed with the pathologic findings in 22 of the 25 patients (88%), were falsely negative in one patient, and falsely positive in two of the patients. In this selected population the most reliable diagnostic tests were CK or CK-MB isoenzymes. The ECG was frequently unhelpful (7 of 25 patients) because of left bundle branch block or paced rhythm, but when unaffected by these depolarization abnormalities it was useful in establishing the correct diagnosis.