This study included 2930 (1642 male, 1288 female) Type 1 (insulin-dependent) diabetic patients diagnosed before the age of 31 years and between 1933 to 1972. The patients were followed from first admission to Steno Memorial Hospital until death, emigration, or until 1 January 1983. Relative mortality was studied, and the influence of calendar year of diagnosis, diabetes duration, age at diagnosis, current age and sex were studied. Relative mortality decreased continuously during the period, and patients diagnosed after 1956 had a relative mortality 30-40% lower than patients diagnosed from 1933 to 1946. Relative mortality increased with increasing diabetes duration until about 20 years of duration, after which it declined. It also increased with increasing age until 31-40 years. It decreased with increasing age at diabetes onset. Factors like calendar year of diabetes onset, age at diagnosis, current age and sex had no influence on relative mortality within the first 15 years of duration, although the relative mortality increased with diabetes duration. In the interval of 16 to 40 years of diabetes duration, the relative mortality decreased with increasing calendar year of diagnosis and age at diagnosis. In patients with a diabetes duration of more than 40 years, the relative mortality decreased with increasing age and diabetes duration. These results show that the prognosis of Type 1 diabetic patients has improved considerably during the last 40 years. Furthermore, they show that diabetes duration is the most important determinant of relative mortality.