Diabetes mellitus has been associated with high mortality rates in patients with acute myocardial infarction (AMI). To better define prognosis in this population, the clinical course of 183 diabetics with AMI was studied. In-hospital mortality for all patients was 28% (52 of 183 patients). Mortality was significantly higher in patients with prior AMI than in patients without prior AMI (41 vs 18%, p less than 0.01) and was significantly higher in women than in men (37 vs 19%, p less than 0.01). The 2-fold increase in mortality among diabetic women was observed both in patients with and without prior AMI. The excess mortality among diabetic women was attributable to their increased risk for severe congestive heart failure (CHF) and cardiogenic shock. Death due to CHF occurred in 22% of all diabetic women with AMI compared with 6% of the diabetic men (p less than 0.01). Death resulting from complications other than CHF was similar for both sexes. There were no male-female differences in the history of prior AMI, systemic hypertension, obesity, nephropathy, frequency of Q-wave AMI, anterior AMI or peak creatine kinase levels to account for the high risk for CHF in diabetic women. It is therefore concluded that diabetic women with AMI are at increased risk for death due to CHF, and that this risk is not readily attributable to known conditions associated with CHF.