The prevalence and features of diabetic retinopathy have been examined in 95 pairs of identical twins, 31 concordant for insulin-dependent diabetes (IDD), 27 discordant for IDD, and 37 concordant for non-insulin-dependent diabetes (NIDD). Optic fundi were examined after pupillary dilatation and retinopathy was classified as nil, background, or severe. In the NIDD twins, 35 of the 37 pairs were in the same category including all but one of the 15 pairs with the same duration of diabetes. However, in insulin-dependent diabetic pairs the situation was quite different; only 21 of 31 pairs were in the same retinopathy category, and of the 10 pairs of co-twins with the same duration of diabetes 5 pairs showed a striking difference for retinopathy. There was no significant difference in the prevalence of retinopathy in the concordant and discordant IDD twin pairs. Diabetic retinopathy was not observed in 32 unaffected co-twins of diabetics. We conclude that genetic factors may be important in the etiology of retinopathy in non-insulin-dependent diabetes but that nongenetic factors must be important in the pathogenesis of retinopathy in insulin-dependent diabetes.