The prevalence and features of diabetic retinopathy have been examined in twenty-three pairs of identical twins—thirteen concordant (both diabetic) and ten discordant (one diabetic, one not)—who have had diabetes for at least fifteen years. In the concordant pairs retinopathy was more common (present in twenty-three out of twenty-six individuals) and more severe (seven blind or partially sighted) and a family history of diabetes was more frequent than in the discordant pairs (retinopathy in five out of ten, none blind). In twelve out of the thirteen concordant pairs the progression and severity of retinopathy was strikingly similar in the two twins and was correlated only with the duration of diabetes. In the thirteenth pair after twenty years of diabetes, one was blind and the other had normal eyes, although they showed no obvious differences in control or other features. It seems that genetic factors may be important in the etiology and time of appearance of diabetic retinopathy.