We describe a 60 year-old woman presenting with visual loss of her left eye. No lymphadenopathies, fever, or weight loss were detected. Neuroimaging studies revealed an extra-axial mass along the posterior aspect of the left optic nerve. The mass was resected and showed xanthomatous histiocytes that were positive for CD-68, occasionally positive for S-100, and negative for CD-1. The lesion was diagnosed as Erdheim-Chester disease (ECD) affecting the CNS. The patient is under systemic evaluation in order to discover other ECD lesions. Microscopic findings and differential diagnoses are discussed.