To elucidate the pathophysiological role of diabetes mellitus in determining the left ventricular regional function of the noninfarcted area, 55 patients with acute Q wave anterior myocardial infarction (MI) were studied. The regional ejection fraction of the noninfarcted area was obtained by radionuclide angiocardiography and was used to estimate the left ventricular regional function of the noninfarcted area. Multiple regression analysis was performed to determine the important variables contributing to the regional ejection fraction based on 10 clinical variables: age, sex, QRS score, diabetes mellitus, hypertension, smoking, postinfarction angina, body mass index, serum cholesterol, and coronary atherosclerosis. A high QRS score (P less than .001) and the association of diabetes mellitus (P less than .05) were the important factors contributing to regional left ventricular dysfunction. The regional ejection fraction and QRS score had an inverse linear relationship in the diabetic and nondiabetic groups, and the regional ejection fraction was significantly lower in diabetic patients at every QRS score (P less than .05). The association of hypertension, severity of coronary atherosclerosis, serum cholesterol level, age, and body mass index did not differ between diabetic and nondiabetic patients, which indicates that diabetes mellitus was not mediated through these atherogenic traits. Thus, diabetes mellitus is another discrete cause of regional left ventricular dysfunction of the noninfarcted area after acute MI.