Optic nerve gliomas are slow-growing tumours most commonly seen in children under 10 years of age. Rapidly progressive proptosis and rapid visual deterioration are uncommon but may occur owing to accumulation of mucoid material, necrosis or hemorrhage. We describe a patient with an optic nerve glioma who manifested sudden proptosis and blindness caused by hemorrhage within the optic nerve sheath. The visual acuity returned to 20/25 after surgical decompression of the nerve and high-dose steroid therapy. Histopathological examination was required to establish the diagnosis of optic nerve glioma with extensive calcification. Optic nerve decompression or short-term high-dose steroid therapy, or both, may be helpful in recovering visual function in selected patients with optic nerve gliomas who have acute visual loss.