BACKGROUND: Information about the occurrence of heart failure in the acute phase of myocardial infarction (MI) in diabetic patients and its impact on prognosis are sparse. AIM: The purpose of the present study was to describe how MI patients with diabetes mellitus (DM) differed from MI patients without DM with respect to the occurrence of heart failure and with respect to the influence of heart failure on mortality during follow-up 30 days extending to 15 years. METHODS: The study is a retrospective long-term follow-up of prospectively recorded data concerning 1954 consecutive cases of MI admitted to one coronary care unit (CCU) between 1979 and 1983. DM was diagnosed in 10% (n=194), with 17% (n=33) on insulin therapy. Patients with DM comprised of a higher proportion of women (DM 36% vs. no DM 26%, P<0.001) compared with non-diabetic patients. Baseline risk factors were more prevalent in the patients with DM. The cumulative incidence of heart failure was higher among patients with than without DM (DM 54% vs. no DM 34%, P<0.001). The incidence of life-threatening arrhythmias were similar in both groups. Only 2% of patients with DM and heart failure survived 10 years of follow-up compared with 15% of the non-diabetic patients with heart failure (P<0.001). In multivariate analysis DM was not independently associated with 30 days mortality. During long-term follow-up DM was an important risk factor for mortality independent on the presence of heart failure. CONCLUSION: DM disposes to the development of heart failure. In acute myocardial infarction diabetic patients with heart failure have a worse prognosis than non-diabetic patients with heart failure.