The aim of the study was to investigate prospectively the prognostic value of blood glucose on admission in diabetic and non-diabetic patients with an acute myocardial infarction. Three hundred and thirty-three diabetic and 565 non-diabetic patients were admitted with acute myocardial infarction during the study period of 3.5 years. There was a significant association between mortality and blood glucose on admission in diabetic patients (regression coefficient, r = 0.92, 0.5 < p < 0.02) but not in non-diabetic individuals (r = 0.69, 0.2 < p < 0.5). Age- and sex-standardized mortality was higher in the diabetic group (12.2% vs 7.4%, p < 0.03), but was identical if standardized also for blood glucose on admission. We conclude that a high blood glucose on admission is a bad prognostic indicator in a diabetic patient with an acute myocardial infarction. The excess mortality in diabetic patients with acute myocardial infarction can be attributed to the higher proportion with hyperglycaemia.