In 787 patients with acute myocardial infarction originally participating in the Göteborg Metoprolol Trial, mortality and morbidity during 5 years' follow-up were assessed and related to whether patients had diabetes mellitus. Diabetes occurred in 78 patients (10%). Patients with diabetes had a different risk factor pattern, including higher age, higher occurrence of angina pectoris and hypertension, whereas smoking habits did not differ. In the early phase (hospitalization), patients with diabetes had a higher mortality (12% versus 8%), required more treatment for heart failure and stayed longer in hospital. Other morbidity aspects, such as severity of pain, occurrence of severe supraventricular and ventricular arrhythmias, high-degree AV-block and infarct size did not differ. During 5 years' follow-up mortality rate in patients with diabetes mellitus was 55% as compared with 30% among patients with no diabetes (p less than 0.001). Reinfarction rate during 5 years was 42% in diabetics versus 25% in non-diabetics (p less than 0.001). In a multivariate analysis, taking into account the differences in risk factor pattern, diabetes turned out to be an independent determinant for long-term mortality and reinfarction (p less than 0.001). We conclude that patients with diabetes mellitus, developing acute myocardial infarction, is a group with particularly high risk of death and reinfarction. Interventions aiming at its reduction have priority.