A 47-year-old woman was referred by her ophthalmologist with a working diagnosis of optic neuritis. She presented with a 6 week history of progressive, painless loss of vision in her left eye associated with complete loss of colour vision. She was otherwise well. On examination she was normotensive. Visual acuity on the right with pinhole was 6/9+3 and 6/0 on the left. Field testing on the right eye was full but in her left eye she was just able to appreciate grey shadows and object outlines. There was a left RAPD and bilateral disc pallor. There was no papilloedema or ophthalmoplegia. There were no other neurological signs or endocrinological abnormalities. Inflammatory markers and an autoimmune screen were normal. Neuro-imaging demonstrated a large 50 × 53 mms mass in sphenoid sinus eroding the skull base into the pituitary floor, the optic canals and displacing both cavernous sinuses. Urgent drainage of this sphenoid mucocele was performed in an attempt to preserve vision in both eyes. In conclusion painless and progressive visual loss is out of keeping for an inflammatory cause. Optic neuritis has a misdiagnosis rate of around 10% and in cases with progressive, painless unrelenting symptoms, prompt neuro-imaging should be considered.