The relationship between the ‘diabetic state’ and massive periretinal proliferation (MPP) in idiopathic retinal detachment was examined. In 18 (3.5%) of 508 cases of idiopathic retinal detachment, MPP occurred before or after retinal detachment surgery. In 24 cases of retinal detachment, a diabetic or suspected diabetic state was found. 11 of the 24 were receiving treatment for diabetes and never developed MPP, while 5 of the 13 patients who were not being treated had MPP. This suggests that MPP in retinal detachment may easily be induced by the diabetic state and prevented by treatment for diabetes. Additional causes of MPP in retinal detachment were giant tears, massive vitreous hemorrhages at operation and frequent recurrence.