To elucidate the factors involved in the reduced survival rate of diabetic patients after acute myocardial infarction (AMI), we prospectively evaluated 100 patients with well-documented diabetes and 426 control patients. We characterized infarct size and analyzed the incidence and severity of congestive heart failure (CHF) and subsequent death with respect to infarct size. The extent of the index infarct was less in diabetic compared to nondiabetic patients, 16.2 ± 2.2 CK-gm-eq/m2 compared with 19.2 ± 0.9 (p < 0.02). However, CHF was more prevalent in diabetic patients (31.2% of the diabetic patients compared to 15.7%). The difference was most prominent in diabetic patients who had sustained prior infarction (50% compared to 16%), but was evident also in diabetic patients with initial infarction (26% compared to 16%). The mortality rate was greater in diabetic patients (p < 0.04). When diabetic and nondiabetic patients were stratified with respect to the presence or absence of CHF, survival curves were comparable. The increased incidence of CHF despite a smaller infarct size suggests that additional factors must contribute to myocardial dysfunction and the resultant excess in mortality.