We retrospectively studied the relationship between the vitreous condition and the development of retinal or optic disc neovascularization in 60 eyes with central retinal vein occlusion associated with extensive retinal ischemia. At initial examination, 24 of 60 eyes (40%) with central retinal vein occlusion had complete posterior vitreous detachment, a prevalence significantly higher than in the fellow eyes. During a mean follow-up period of 43 months, 13 of 34 eyes (38%) without posterior vitreous detachment developed retinal or disc neovascularization, or both, but none of the 24 eyes with complete posterior vitreous detachment developed neovascularization (P less than .001). Our results suggest that central retinal vein occlusion may induce the development of posterior vitreous detachment, and that posterior vitreous detachment may play a role in protecting eyes with central retinal vein occlusion from posterior segment neovascularization.