The prevalence of microvascular and acute diabetic complications, and their relation to duration of diabetes and glycaemic control were examined in a cross-sectional study of 3250 IDDM patients in Europe (EURODIAB IDDM Complications Study). Mean (SD) duration of diabetes was 14.7 (9.3) years. HbA1c and AER were measured centrally. Retinopathy was assessed by centrally graded retinal photography. Autonomic neuropathy was measured by heart rate and blood pressure responses to standing up. Sensory neuropathy was measured by biothesiometry. Normal HbA1c was found in 16% of patients. An AER of 20 micrograms/min or higher was found in 30.6% (95% CI 29.0%, 32.2%) of all patients, and 19.3% (15.6%, 23.0%) of those with diabetes for 1 to 5 years. The prevalence of retinopathy (46% in all patients; 82% after 20 or more years) was substantially lower than in comparable studies. Of all patients 5.9% (5.1%, 6.7%) had postural hypotension, 19.3% (17.9%, 20.7%) had abnormal heart rate variability, 32.2% (30.6%, 33.8%) reported one or more severe hypoglycaemic attacks during the last 12 months and 8.6% (7.6%, 9.6%) reported hospital admission for ketosis over the same period. Microvascular and acute complications were clearly related to duration of diabetes and to glycaemic control. However, the relation of glycaemic control to raised albuminuria differed qualitatively from its relation to retinopathy.