All 906 patients with insulin-dependent diabetes mellitus (IDDM) diagnosed before the age of 31 years, prior to 1943, and admitted to the Steno Memorial Hospital were followed until death or until 1 January 1984. In an attempt to identify factors of prognostic importance, we compared patients dying within 35 years of the onset of diabetes with patients surviving for 40 years or more. Three hundred and seventy-seven patients survived for 40 years or more; of these 224 were still alive and invited to a re-examination, in which 184 participated. After 40 years of diabetes, the most frequent complications were impaired vision (due to diabetic retinopathy) and persistent proteinuria. However, 53% had no major complications despite 40 years with IDDM. The 184 re-examined patients (median age 60 years, median diabetes duration 47 years) were all genuine IDDM patients, as defined by stimulated C-peptide levels. Proliferative retinopathy or visual impairment was found in 56% of the 184 patients, abnormal ECG or amputations in 26%, and elevated urinary albumin excretion rate (AER) greater than or equal to 30 mg/24 h) in 45%. Twenty-five per cent had none of these complications. Proliferative retinopathy was associated with elevated AER and raised systolic blood pressure, macroangiopathy with the use of antihypertensive drugs, and proteinuria with low age at diagnosis, large increase in systolic blood pressure, smoking, and insulin-binding antibodies. Sex, age and diabetes duration were not associated with any of these three late diabetic complications.