The development of an avascular filtering bleb following trabeculectomy with mitomycin-C is a frequently encountered clinical phenomenon. We studied a small portion of conjunctiva from such a filtering bleb with light and electron microscopy. Examination revealed markedly irregular epithelium. The substantia propria contained only a few cells and no vessels. The basement membrane of the basal layer of the conjunctival epithelial cells was irregular with areas of variable thickness, breaks, and complete absence. These factors may contribute to the long-standing hypotony following trabeculectomy with mitomycin-C seen in a considerable number of patients.