The development of retinopathy was examined over an 8-year period in 56 patients with insulin-dependent diabetes (IDDM) who were under the age of 25 when they were diagnosed as having diabetes and who had simple retinopathy in 1980 (mean duration of diabetes, 20.5 years [1988]; mean level of HbA₁ 11.4%[1980-1988]). The duration of diabetes among the 12 patients who had developed proliferative retinopathy after 8 years was 22.2 years, compared with 20.7 years in the 33 patients with simple retinopathy and 17.8 years in the 11 patients without retinopathy. Life-table analysis was performed to determine the duration of diabetes until the retinal lesions became proliferative. Proliferative retinopathy occurred in those with over a 10-year history of diabetes. The interval from diagnosis of IDDM to 25% risk for developing proliferative retinopathy was 18.9 years, and that to 50% risk was 27.6 years.