Background: Acute optic neuropathy due to an intracranial lesion may masquerade as optic neuritis or nonarteritic anterior ischemic optic neuropathy (NAION). We reviewed the records of patients who presented with acute unilateral optic neuropathy that was initially diagnosed as optic neuritis or NAION but who ultimately proved to have an underlying structural lesion. Methods: Retrospective observational case series. We reviewed the records of patients with the initial diagnosis of optic neuritis or NAION in whom the diagnosis was changed to an intracranial etiology at four tertiary care neuro-ophthalmology centres between 1995 and 1998. Results: Eight cases were identified in which atypical features prompted further investigation, including neuroimaging, leading to the diagnosis of an intracranial etiology for the optic neuropathy. Five patients were discovered to have neoplasms (a tuberculum sellae meningioma in two cases, an optic nerve sheath meningioma in two cases and a metastatic lesion in one case), and three patients had intracranial sarcoidosis. Atypical features for optic neuritis included a progressive course, absence of pain, optic atrophy at presentation, lack of significant visual improvement and age over 40 years. For NAION, the atypical features included progressive course, optic atrophy on presentation, absence of vasculopathic risk factors and preceding transient visual loss.