The aim of the present study was to elucidate the effect of diabetes and metabolic control on the presentation, sources, pathogens and outcome of common infections. Of 515 patients admitted to three departments of internal medicine because of a suspected acute infection, 132 (26%) had diabetes mellitus. Osteomyelitis was diagnosed in 3% of the diabetic patients and in 1% of patients without diabetes, and infection of the extremities in 7% and 0%, respectively (p = 0.003). Klebsiella sp. caused 24% of urinary tract infections in diabetic patients, versus 13% in patients without diabetes (p = 0.1). The percentage of Staphylococcus aureus infections in diabetic patients was 10% versus 5% in non-diabetic patients (p = 0.06). The gross mortality rate in the diabetic patients was 10%, and in patients without diabetes, 12%. In patients without fatal underlying disorders, mortality in the diabetic patients was 10% (2% in patients with glycosylated haemoglobin (GHb) lower than median, and 17% in patients with GHb higher than median) and in the non-diabetic patients 4% (p = 0.04). Five factors were independently and significantly related to mortality in diabetic patients: acute respiratory distress (very large odds-ratio [OR]), coma (OR 3.8, 95% confidence interval [CI] 1.0-14.3), GHb above the median (OR 3.3, 95% CI 1.8-6.2), the interaction between GHb and absence of a severe underlying disorder (OR 12.0, 95% CI 2.9-50.7) and duration of diabetes (OR of 1.072 for 1-year increment, and 1.42 for a 5-year increment). Choice of empiric antibiotic treatment in diabetic patients with suspected bacterial infection should take into account the preponderance of Klebsiella sp. and Staphylococcus aureus infections. The present results favour an association between poor glycaemic control and a fatal outcome of infectious diseases in diabetic patients.