The incidence of nephropathy in Type 1 diabetes mellitus has declined during the past decade, probably as a result of improved glycaemic control. We wanted to investigate whether the incidence of severe retinopathy has changed during the same time period and to evaluate the importance of glycaemic control in relation to the development of severe retinopathy and nephropathy. All 213 patients in whom Type 1 diabetes mellitus was diagnosed before the age of 15 years between 1961 and 1980 in a district in southeastern Sweden were studied. Ninety-two per cent of the patients were followed from the onset of diabetes to 1991 or to death. The cumulative incidence of severe retinopathy was not significantly different between the patients with diabetes onset 1961-65, 1966-70, 1971-75, and 1976-80. The risk of developing severe retinopathy or nephropathy was higher in patients with very poor glycaemic control (HbA1c > or = 8.4%) vs patients with poor control (HbA1c > or = 7.2 < 8.4%; p < 0.001). Patients with poor control had an increased risk of developing severe retinopathy vs patients with good control (HbA1c < 7.2%; p < 0.008) but there was no difference in the risk of nephropathy. No patients with good control developed nephropathy and only one patient developed severe retinopathy during 25 years of diabetes.