Neuromyelitis optica (NMO) is an inflammatory/demyelinating disorder predominantly affecting the optic nerves and spinal cord. Recent findings showed an underlying humoral abnormality in NMO, characterized by a serum antibody against aquaporin-4 (Aqp-4-Ab). In this study, we evaluated the Aqp-4-Ab status among Turkish patients with NMO to determine the clinical and prognostic relevance. Serum samples from 35 consecutive patients with NMO followed at a single center and diagnosed according to the 2006 revised criteria, were evaluated for Aqp-4-Ab. All samples were obtained during a relapse prior to any immunosuppressive treatment. Aqp-4-Ab was positive in 21/35 (60%) patients. Among these cases, 11 had an EDSS of 6.0 or more, whereas only two patients in the seronegative group had such severe disability (p