Diabetes mellitus is believed to increase susceptibility to infectious diseases. The effects of hyperglycaemia per se on infectious disease risk are unknown and the influence of diabetes on infectious disease outcome is controversial. We studied 10,063 individuals from the Danish general population, who were participants in The Copenhagen City Heart Study, over a follow-up period of 7 years. Risk of hospitalisation caused by any infectious disease, and subsequent risk of disease progression to death were estimated by Cox proportional hazards regression analysis. At baseline, 353 individuals reported having diabetes. During 71,509 person-years of follow-up, a total of 1,194 individuals were hospitalised because of an infection. The risk of pneumonia (adjusted hazard ratio [aHR] 1.75, 95% CI 1.23-2.48), urinary tract infection (aHR 3.03, 95% CI 2.04-4.49) and skin infection (aHR 2.43, 95% CI 1.49-3.95) was increased in subjects with diabetes compared with subjects without. Each 1 mmol/l increase in plasma glucose at baseline was associated with a 6-10% increased relative risk of pneumonia, urinary tract infection and skin infection after adjustment for other possible confounders. Among patients hospitalised for urinary tract infection, diabetic patients were at an increased risk of death at 28 days after admission compared with non-diabetic subjects (HR 3.90, 95% CI 1.20-12.66). In the Danish general population, diabetes and hyperglycaemia are strong and independent risk factors for hospitalisation as a result of pneumonia, urinary tract infection and skin infection. Further, diabetes has a negative impact on the prognosis of urinary tract infection.