Vitrectomy may prevent the progression of an impending macular hole by removing the layer of cortical vitreous from the posterior retina. To determine the nature of the cortical vitreous tissue, we identified and removed from the surface of the posterior retina a thin sheet of what appeared to be posterior cortical vitreous in 29 patients undergoing vitrectomy for an impending macular hole. In seven patients, the tissue was isolated for transmission electron microscopic study. Millipore filter specimens of the vitreous aspirates from all of the patients were studied by light microscopy. Vitreous condensates were present in all 29 specimens, fibrocellular membrane fragments were present in three, and fragments of internal limiting membrane were present in four. A collagen matrix was present in each of the seven specimens studied by electron microscopy, and in every specimen, the collagen's diameter was consistent with indigenous vitreous collagen. These findings confirm the presence of an acellular tissue layer on the posterior retina in eyes with an impending macular hole and indicate that it is usually indigenous vitreous collagen.