A 34-year-old man suffered a 2-week history of deteriorating visual loss without pain in the left eye. Orbital magnetic resonance imaging revealed a 24 × 28 × 28 mm mucocele formation in the left posterior ethmoid sinus, with bony remodeling and compression of the left orbital apex. The patient underwent functional endoscopic sinus surgery; however, the visual deficit did not improve within the 6-month follow-up period.