The authors evaluated the effect of posterior vitreous detachment (PVD) in 172 eyes (114 patients) with nonproliferative diabetic retinopathy on the development of retinal or optic disc neovascularization. Patients were followed for more than 6 months after the initial vitreous examination (mean, 32 months). None of the patients 39 years of age or younger had PVD, but this condition became more prevalent thereafter with age. In 124 eyes of patients 40 years of age or older, neovascularization developed eventually in 20 (22%) of 93 eyes without PVD but in only 1 (3%) of 29 eyes with complete PVD (P less than 0.05). A vitreous examination was invaluable in predicting the development of retinal or optic disc neovascularization in diabetic patients 40 years of age or older.