We analyzed retrospectively the clinical course and prognosis of 565 consecutive patients with acute myocardial infarction (AMI), 117 of them with a previous history of diabetes mellitus. Male/female ration was 7.9/2.1 in non diabetics and 7.0/3.0 in diabetics (p < 0.03). Incidence of hypertension and hyperlipidemia was higher in diabetic patients as well as history of congestive heart failure (13.7% vs 6.5 in non diabetics p < 0.01). The type and location of AMI did not differ among groups, however the incidence of congestive heart failure Killip class III-IV was higher in diabetic patients (31.6 vs 21.2%). Peak CPK values were lower in diabetics (1.270 +/- 1.179 vs 1.648 +/- 1.377 U/l p < 0.01). Cardiac mortality was higher one month and one year after AMI in diabetics (17.1 vs 13.6% and 21.4 vs 17.8% respectively, p < 0.01). Univeriate and multivariate analysis identified new bundle branch block, heart failure and advanced age as independent predictors of mortality in both groups of patients. It is concluded that the worst prognosis of diabetic patients with AMI may be related to a previously depressed ventricular function and that appropriate metabolic control and treatment of associated risk factors, could improve the prognosis of diabetics patients with AMI.