A 59-year-old man undergoing chemoradiotherapy with temozolomide according to the Stupp et al protocol1 (75 mg/m2/d continuously during radiotherapy followed by 6 cures of 5 days of temozolomide 200 mg/m2/d) for a high-grade cerebral astrocytoma located at the left parieto-temporo-occipital junction presented with rapidly progressive, painless loss of vision of the left eye for a few weeks. Visual acuity was hand motions. A relative afferent pupillary defect was observed. Anterior segment examination was normal. Ophthalmoscopy (Figure, A) revealed a massive, whitish tumor infiltrating the optic nerve associated with cellular seeding in the overlying vitreous. The right eye was normal. Intraocular extension of a cerebral glioblastoma is rare and associated with poor vision and survival.2 Glioblastoma located primarily in the optic nerve has been reported in fewer than 50 cases.3 Testing for the IDH-1 protein mutation, known to be correlated with larger glioblastomas with more satellite lesions, was negative in this patient.