In 168 eyes with preretinal membranes studied histopathologically, five major distinct types of membranes were categorized: 61 glial, 28 fibrous, 22 cortical vitreous, 16 retinal pigment epithelium, 15 fibroinflammatory, 24 combinations, and one endothelial and melanomatous membrane each. The overall prevalence of preretinal membranes was 5.5% in 2,000 cases studied. Selected cases examined by electron microscopy were supportive of the origin of the cells in the preretinal membranes in the glial, fibrous, and retinal pigment epithelial types. Glial preretinal membranes occurred as the result of defects in the internal limiting membrane, such as retinal pits or holes, and from the optic nerve head in association with posterior vitreous detachment. Fibrous preretinal membranes were associated with proliferative retinopathy and long-standing retinal detachment. Studies of the cellular origin of these membranes were inconclusive. The cortical vitreous type of preretinal membrane was hypocellular and occurred in the absence of associated ocular abnormalities. Pigment epithelial preretinal membranes occurred in eyes with rhegmatogenous retinal detachment.