A 71-year-old woman developed chronic progressive visual loss in the right eye and computed tomographic scan showed enlargement of the intraorbital optic nerve consistent with optic nerve sheath meningioma. Over 12 years, the contralateral optic nerve was not clinically affected, and serial neuroradiologic imaging showed no evidence of intracranial tumor extension. Death occurred from metastatic adenocarcinoma of the lung 14 years after initial visual loss. Examination of the postmortem specimen of optic nerve and chiasm revealed extradural extension of meningioma with spread to the region of the optic chiasm and hypothalamus. A large focus of metastatic adenocarcinoma was present within the intraorbital portion of the meningioma. Carcinoma metastatic to intracranial meningioma is rare; to our knowledge, this is the first reported case in an optic nerve sheath meningioma. Neuroimaging may be inadequate to predict the value of tumor excision in preventing intracranial spread of optic nerve sheath meningioma.