A case of a 6‐year‐old girl with unilateral retinoblastoma with widespread dissemination into the subarachnoid space through the optic nerve is reported. The enucleated right eye showed advanced tumor growth characterized by exo‐ and endophytic invasion. The growth infiltrated through the lamina cribrosa and directly into the substance of the optic nerve. Extension into the central nervous system was through the optic nerve. The tumor consisted of only anaplastic cells and showed no tendency of rosette formation. Electron micrographs of the surgical specimen and necropsy material fixed with formalin suggested that tumor cells may be of primitive and anaplastic without any differentiation to glial or neuronal cells. The tumor is consistent with udnifferentiated retinoblastoma previously reported by Reese