Neuromyelitis optica (NMO; Devic's Syndrome) is an idiopathic, often relapsing, severe inflammatory disorder preferentially affecting optic nerves and spinal cord. The distinction of NMO from multiple sclerosis (MS) as a separate disease entity has been controversally discussed for a long time. Though both diseases show demyelinisation, they differ in typical clinical, imaging and immunopathological findings. The recent identification of serum aquaporin (AQP)-4 antibody clearly separates NMO from MS. In addition, the identification of AQP-4 antibodies supports peripheral humoral autoimmune pathogenesis in NMO and permits early initiation of effective therapy for prevention of attack-related disability.