A case of bilateral retrobulbar optic neuritis with profound visual loss is described. Magnetic resonance imaging (MRI) with fat-saturation suppression and gadolinium enhancement done 15 days after the onset of symptoms demonstrated bilateral posterior optic nerve disease in a pattern similar to that seen in previously reported cases. However, subsequent retrospective review of an MRI study done at a different hospital during the first week of disease demonstrated a single focus of gadolinium enhancement within the body of the chiasm. This observation suggests that bilateral demyelination of the posterior optic nerves may begin as a single focus of chiasmal disease that then spreads to involve the proximal portions of the optic nerves.