The relative importance of duration of diabetes before and after 13 yr of age as a risk factor for retinopathy was investigated using data from 200 persons who were younger than 26 yr of age. These persons were identified in a population-based study of diabetic retinopathy in southern Wisconsin in 1980–1982. Retinopathy was found in 9% of persons who were younger than 13 yr and in 34% of persons who were 13 yr or older and had been diagnosed at or after 13 yr. Presence of retinopathy was more strongly associated with the duration of diabetes after 13 yr of age than before it.