Population-based epidemiologic data on the incidence and progression of diabetic retinopathy are important in medical counseling and rehabilitative services and for developing approaches to preventing diabetic retinopathy. We performed a population-based study in southern Wisconsin of insulin-taking diabetic persons diagnosed before 30 years of age. Of the 271 who had no retinopathy at the first visit, 160 (59%) developed it by the time they were reexamined four years later, and 75 (11%) of the 713 free of proliferative diabetic retinopathy developed it. Overall, worsening of retinopathy occurred in 41% of the population, whereas improvement occurred in only 7%. The incidence of proliferative retinopathy rose with increasing duration until 13 to 14 years of diabetes, thereafter remaining between 14% and 17%. These incidence data underscore the need for careful ophthalmologic follow-up of these people.