Congenital cystic lesions of the optic nerve are exceedingly rare; only one case is reported in the world literature. The authors describe a case of congenital simple glial cyst in the intraorbital portion of the optic nerve with a brief review of the literature and comment on its histogenesis. A 45-day-old male infant was admitted to the hospital because of progressive proptosis and hypotropia in the left eye, which had been present since birth. Magnetic resonance imaging of the left orbit revealed an ovoid, well-demarcated, homogeneous cystic mass in the intraconal retrobulbar area. The mass compressed the left eyeball with downward and lateral displacement. The wall of the cystic mass was very thin, and a needle puncture of the cyst released clear, colorless, watery fluid. The cystic wall was lined by loose astroglial nerve fibers with some scattered glial cells.