A study was made of diabetic retinopathy in acromegaly. 10 of 15 patients with acromegaly had diabetes mellitus, and 3 of the 10 showed diabetic retinopathy. 2 of them had a diabetic family history. 1 patient with a diabetic family history had retinopathy of state IIIa in Scott's classification, and the other 2 showed a few microaneurysms and/or punctate hemorrhages in the macula. Diabetes mellitus and diabetic retinopathy in acromegaly showed no correlation with the duration of acromegaly and diabetes mellitus, age, or growth hormone level. No diabetic cataract was found in the present series. It was concluded that diabetic retinopathy due to secondary diabetes mellitus is usually slight or moderate. Diabetes mellitus with severe retinopathy is probably primary diabetes due to a genetic defect, and secondary diabetes may be different in nature from the primary disease.