We report a rare case of massive retinal gliosis that developed in a 32-year-old woman who had been born with bilateral microphthalmia. The patient had recently noticed left ophthalmos and underwent total resection of the affected eyeball. Histologically, the vitreous body had been totally replaced by massively proliferated spindle cells, which had delicate fibrillary cytoplasm without nuclear atypia. Because the attenuated retinal pigment epithelium and intact sclera were preserved at the periphery of the tumor, the tumor was thought to be retinal in origin. Immunohistochemically, the spindle cells were strongly positive for glial fibrillary acidic protein and neuron-specific enolase and partly positive for S-100 protein. These findings led to a diagnosis of massive gliosis of the retina. Clonality analysis of the tumor using a human androgen receptor assay revealed the polyclonal nature of the proliferating spindle cells. This is the first documentation of the polyclonality of this disease.