Seven eyes and one vitrectomy specimen from seven patients with pars planitis and complications such as secondary glaucoma and phthisis bulbi were studied histopathologically. Two of the specimens were also examined by electron microscopy. All cases showed a typical intravitreal "snowbank" opacity overlying the pars plana. On light microscopy, these "snowbanks" appeared to consist of a loose fibrovascular layer containing occasional fibrocyte-like cells and scattered mononuclear inflammatory cells adjacent to the hyperplastic nonpigmented epithelium of the pars plana. Within the vitreous base, an extensive fibroglial proliferation had developed, often drawing the peripheral retina anteriorly into the "snowbank." Electron microscopy showed this fibroglial tissue to be composed of condensed vitreous collagen and probable fibrous astrocytes which had produced larger-diameter (about 24 mm) collagen fibrils. The fibroglial proliferation also appeared continuous with an ultrastructurally similar preretinal fibroglial membrane. All eyes showed prominent lymphocytic cuffing and mural infiltration of retinal veins, with sparing of the arterioles. Several cases showed cystoid macular edema. Only mild choroiditis or cyclitis could be shown in some cases. We believe that in pars planitis the fibroglial "snowbank" may reflect a common inflammatory process involving both the peripheral retina and vitreous base.