We followed 1, 134 patients with Type 1 (insulin-dependent) diabetes, diagnosed between 1933 and 1952, until 1982 or death or until their emigration. Their age at onset of diabetes was under 31 years. Information concerning the development of persistent proteinuria was sought in every case. In 104 cases, the data were either questionable or the patient could not be traced. Twenty-nine patients developed non-diabetic proteinuria. Among the remaining 1,001 patients, 406 developed persistent proteinuria (350 died) and 595 did not (166 died). The incidence of persistent proteinuria was highest among men; it decreased with increasing year of diabetes onset from 1933 to 1952, and decreased with increasing age at onset. The relative mortality was extremely high among patients with persistent proteinuria, increasing to a maximum of about 100 at age 35 years. Patients not developing proteinuria had a relatively constant low relative mortality of about 2. The decreasing incidence of persistent proteinuria and the decreasing mortality with increasing calender year of diabetes onset resulted in a 50% increase in life-expectancy among patients diagnosed in 1950 compared with patients diagnosed in 1935. In patients who developed persistent proteinuria, relative mortality was higher in women than men at all ages. In patients who did not develop proteinuria, relative mortality was similar in men and women after the age of 35. Uraemia was the main cause of death in patients with persistent proteinuria, although cardiovascular deaths were more frequent than in patients without proteinuria. Thus, proteinuria is associated not only with death from uraemia but also from cardiovascular disease. It is concluded that the development of persistent proteinuria is a major life-threatening complication in patients with early-onset Type 1 diabetes. Patients who do not develop proteinuria have almost a normal life expectancy.