BACKGROUND: Antibodies to aquaporin-4 (AQP4-Ab) are found in 60-80% of patients with neuromyelitis optica (NMO), a severely disabling inflammatory CNS disorder of putative autoimmune aetiology, which predominantly affects the optic nerves and spinal cord.OBJECTIVE: To assess the frequency of AQP4-Ab in patients with optic neuritis (ON), and to investigate the prognostic implications of AQP4-Ab seropositivity in such patients.