Both Müller cells and astrocytes have been implicated in the dispute over the histogenesis of massive retinal gliosis. We studied three cases of massive retinal gliosis by light and electron microscopy and immunocytochemistry. Spindle fibrillary glial cells were joined by zonulae adherentes resembling those of the external limiting membrane of the retina. Furthermore, these cells produced a continuous basement membrane around an extracellular space filled with fine filaments, which was highly suggestive of vitreous cavity. In the proliferating cells, immunoperoxidase technique disclosed the presence of carbonic anhydrase isoenzyme C, characteristically found only in Müller cells. The glial cells in the preretinal membrane away from the gliotic nodule showed similar characteristics. We concluded that both the nodule of massive retinal gliosis and the associated preretinal glial membrane resulted from the proliferation and migration of Müller cells.