Juvenile pilocytic astrocytoma of the optic nerve (glioma of childhood) usually grows slowly. Rapidly evolving proptosis is rare and reflects the accumulation of mucinous or hemorrhagic material within the tumor. Rapid proptosis is more common in those patients beyond the first two decades of life who have long-standing tumors. We studied a 26-year-old woman with rapid proptosis caused by a large blood-filled cyst in a pilocytic astrocytoma of the optic nerve. The cyst was surgically removed along with the intraorbital portion of the optic nerve.