Clinical and electron microscopic studies of an idiopathic epiretinal membrane that was nonvascularized and progressive showed three types of cells: (1) predominant, fibrocyte-like cells; (2) macrophage-like cells; and (3) glial cells. Most of the abundant collagen showed characteristics of normal vitreous fibrils, although thicker collagen fibrils were also present. These ultrastructural findings, when compared with previously reported features of simple and secondary epiretinal membranes, suggest that clinically complicated epiretinal membranes are composed of more than one cell type, whereas simple epiretinal membranes consist only of glial cells. The fibrocyte-like and macrophage-like cells seen in our case were interpreted as being hyalocytes, rather than being derived from glial or retinal pigment epithelial cells. We support this interpretation by the ultrastructural features of the cells, the presence of abundant normal vitreous collagen, the biologic characteristics of hyalocytes as described by other investigators, and the absence of the ultrastructural features of glial or retinal pigment epithelial cells.