Background: Neuromyelitis Optica (NMO) is an autoimmune inflammatory disease of the central nervous system (CNS), disturbing spinal cord and optic nerves specified by the presence of pathogenic serum autoantibodies against aquaporin 4 (AQP4) in the majority of patients. The role of T-cell is ambiguous. OX40 (CD134) is member of the tumor necrosis factor receptor family and is expressed selectively on activated T lymphocytes which increases in several autoimmune diseases. The aim of the study was to evaluate serum OX40 levels in patients with NMO.Methods: The study involved 19 patients with NMO and 19 controls. Serum OX40 levels were determined by the enzyme-linked immunosorbent assay (ELISA).Findings: The present study showed no significant different between OX40 levels in the serum of patients with NMO compared with controls (P=0.378). Serum level of OX40 in women were lower than men (P=0.470). In addition, we found that median serum OX40 levels in AQP4 Ab-serpositive were higher than seronegative NMO patient but was not statistically significant (P=0.410).Conclusion: Serum OX40 levels could not be used as a diagnostic or treatment marker for NMO.