A prospective study of acute myocardial infarction was carried out in 1239 patients in order to assess both the prognostic significance of diabetes mellitus and the clinical characteristics associated with age and gender. Diabetes mellitus (DM) was found in 386 cases, often associated with old age, female gender, and more prevalent history of angina, heart failure, and hypertension. DM patients were admitted later and they were less likely to receive thrombolytic therapy, 47.9 vs. 58.1% (P<0.001). Complications more often associated with DM were: heart failure, 45 vs. 24.5% (P<0.01), and early, in-hospital and 1-year mortalities, 7.2 vs. 3.9% (P<0.05), 17.6 vs. 9.1% (P<0.001), and 29.2 vs. 16.2% (P<0.001), respectively. Compared with diabetic men, diabetic women were older and had a more prevalent history of hypertension and congestive heart failure. Diabetic women also had a higher rate of heart failure during hospitalisation, and of mortality, than diabetic men: early: 11.7 vs. 4.5% (P<0.01); in-hospital: 29.6 vs. 10.3% (P<0.001); and 1-year: 42.7 vs. 21.1% (P>0.001). DM was not selected by the multivariate analysis as a variable with independent prognostic value for mortality. In separate multivariate analysis for diabetic and non-diabetic patients, female gender had independent prognostic value for mortality only in the case of the diabetic population.