To study objectively the epidemiology of retinopathy in juvenile-onset diabetes, we performed fundus photography and fluorescein angiography, using the Diabetic Retinopathy Study protocol, on 122 juvenile diabetics and 65 demographically similar non-diabetic subjects as the control group. Photographs and angiograms were masked as to subjects' identities and evaluated independently by five retinal subspecialists. There was no retinopathy in control subjects. In diabetics, prevalence of retinopathy increased with the duration of disease, being 0% after zero to four years, 27% for five to nine years, and 71% for more than ten years. Retinopathy also increased in prevalence with age with a sharp rise after age 15. There is indication that age and duration act independently. Details of our method for establishing the diagnosis of diabetic retinopathy are presented, together with the degree of observer variability in identifying early lesions.