Characterization of glial involvement in proliferative diabetic retinopathy. We studied the relationship of glial cells and other supporting tissues associated with newly formed blood vessels in proliferative diabetic retinopathy. Seventeen postmortem, freshly enucleated eyes from diabetic patients and 34 epiretinal and preretinal membranes removed during vitreous surgery for proliferative diabetic retinopathy were analyzed using the peroxidase-antiperoxidase method for light microscopy and protein A/gold labeling of ultrathin cryosections for transmission electron microscopy in addition to routine transmission and scanning electron microscopy. We found that glial cells are commonly and characteristically found in elevated diabetic proliferations and present at the vitreous surface. The newly formed blood vessels, however, were not seen at the edge of elevated epiretinal and preretinal membranes in early and intermediate stages. These results suggest that glial cells may extend beyond the vascularized areas of the proliferative tissue. It is possible that glial cells and their extracellular matrix contribute to the framework leading to the development of new blood vessels. 