Thirty preretinal membranes excised from diabetic patients during pars plana vitrectomy were examined by transmission electron microscopy, scanning electron microscopy, or both. All the specimens were still correctly orientated after removal from the eyes. All the specimens contained collagen in varying amounts and most had blood vessels with a thickened basal lamina and adjacent pericytes. Fenestrated capillaries were identified in only four of the 21 membranes containing blood vessels. A wide variety of cell types were scattered within the membranes, including polymorphonuclear leukocytes, lymphocytes, plasma cells, macrophages (containing melanin and hemosiderin), glial cells, retinal pigment epithelium, and cells of uncertain derivation. Scanning electron microscopy showed the retinal side of the membranes to be smoother than the vitreal side and disclosed preretinal cells on the vitreal side of the membranes, either as individual stellate cells or as a monolayer of flattened cells with microvilli. Membranes stripped from the surface of the retina had a smooth, generally acellular surface on the retinal side.