A review of 484 consecutive eyes that were undergoing an initial pars plana vitrectomy for the sequelae of proliferative diabetic retinopathy disclosed that 41 eyes (8.5%) required one or more additional vitrectomy operations. The primary causes for reoperation included rhegmatogenous retinal detachment in 18 (44%) of the 41 eyes, recurrent vitreous hemorrhage in 21 eyes (51%), and glaucoma in two eyes (5%). The visual prognosis was worse in the group with rhegmatogenous retinal detachment, with 10 (56%) of 18 eyes progressing to no light perception (P = .003). Severe preretinal and subretinal fibrous proliferation, as demonstrated histopathologically, accounted in large part for the poor result. The preretinal membrane formation appeared to occur secondary to a combination of diabetic extraretinal vascular growth and proliferative vitreoretinopathy. Among the total group of 41 eyes that required subsequent surgery, the retina eventually remained detached in 18 eyes (44%), and phthisis bulbi occurred in 13 eyes (32%). Rubeosis iridis developed in 17 (94%) of 18 eyes in which the retina remained detached.