The occurrence of diabetes mellitus and its complications and prognosis in an unselected consecutive series of patients with myocardial infarction (MI) was studied. Out of 341 patients 81 (24%) had diabetes. Comparisons were made between patients with and without diabetes. Age was higher and female sex more common among patients with diabetes. A considerable proportion of patients with diabetes were on digitalis when admitted (51%) compared to 20% of those without. Ventricular tachyarrhythmias requiring treatment did not differ between the two groups. High-degree AV-block was considerably more common among patients with diabetes (19%) than those without (7%; P less than 0.001). Mortality was higher in diabetic than in non-diabetic patients both during the hospital phase (25%, vs. 16%; P less than 0.02) and during one year of follow-up (53% vs. 28%; P less than 0.001). Diabetes was an independent prognostic risk factor for death (P less than 0.01). Fatal reinfarction was more common among diabetic patients (30%) than those without (14%; P less than 0.05). In conclusion diabetics with MI have a poor prognosis despite improvements in coronary care. The high late mortality is to a large extent related to a high proportion of fatal reinfarctions.