This article reports the first known case of permanent blindness due to irreversible unilateral optic neuritis (ON) related to infliximab (Remicade) treatment of a patient with ulcerative colitis. A young male, with a family history of inflammatory bowel disease, was diagnosed with ulcerative colitis at the age of 20. He was treated with steroids and a 5-aminosalicylic acid drug without considerable effect, and later admitted to our hospital due to a relapse during reduction of the prednisolone dosage. A new colonoscopy showed moderate ulcerative colitis activity and the patient was declared as a steroid nonresponder. A treatment of 400 mg intravenous infliximab was initiated along with 150 mg/day of azathioprine (Imurel). Three days after the second infliximab treatment the patient woke up with no vision on the left eye and with pain during ocular movement. Brain and orbitae magnetic resonance imaging showed ON on the left optical nerve without any abscess or thrombosis. The patient was treated with 1000 mg methylprednisolone (Solu-Medrol) intravenous for 3 days and afterward with 75 mg prednisolone orally without any effect. At the 3-month follow up, the patient's vision had not improved, and he was declared permanently blind on the left eye. A neurologist also examined the patient, but no abnormality or cause of the ON was found.