We studied the ultrastructural characteristics of 23 vascularized preretinal membranes removed from 22 eyes during pars plana vitrectomy for proliferative diabetic retinopathy. Traction detachment of the macula was the most frequent reason for surgery (15 of 22 eyes), followed by nonclearing vitreous hemorrhage (seven of 22 eyes). Vessels found in the membranes were studied by serial sectioning and categorized as having either developing, mature, or regressing characteristics. Developing vessels were seen in ten of 23 membranes (43%), mature vessels in 19 of 23 membranes (83%), and regressing vessels in 13 of 23 membranes (57%). Endothelial fenestrations and cell separations were rare. With three-dimensional reconstruction, we found that new vessels often extended cytoplasmic processes into the extracellular matrix and that lumina were present even at the distalmost tips of the vessels. Solid cords of endothelial cells were not seen. We concluded that in proliferative diabetic retinopathy, new vessels develop by a process of focal extracellular matrix degradation, generalized and exuberant extracellular matrix production, cytoplasmic microvillus extension into the extracellular matrix, and active lumen formation.