Background: Oxidative stress is involved in pathogenesis of some psychiatric disorders. To examine the role of oxidative stress in the etiopathogenesis of obsessive-compulsive disorder (OCD), we aimed to determine oxidative stress indices, including malondialdehyde (MDA) levels in serum and red blood cells (RBC) membrane, total antioxidant capacity (TAC), serum glutathione (GSH) levels, serum antioxidant vitamins (A and E), and Na + /K + -ATPase activity, in patients with the mentioned disorder vs. healthy controls., Method: 39 OCD patients diagnosed based on Diagnostic and Statistical Manual of Mental Disorders (DSM-V) and 39 volunteer healthy subjects were included in this study. MDA levels in serum and RBC membrane were measured using fluorometric method. Serum TAC level, serum GSH level, and Na + /K + -ATPase activity were also measured using spectrophotometric methods. Serum levels of vitamins were calculated by reversed-phase high-performance liquid chromatography (RP-HPLC)., Result: There was a significantly higher MDA level in serum ( p < 0.0001) and RBC membrane ( p = 0.002) of OCD patients compared with those in controls. A significant reduction in vitamin A ( p = 0.001) and vitamin E ( p = 0.024) levels was found in OCD patients vs. controls. There was significantly lower activity of erythrocyte membrane Na + /K + -ATPase in RBC membrane of OCD patients vs. controls ( p < 0.0001)., Conclusion: Our findings indicate significantly higher levels MDA in both serum and RBC membrane, lower levels of serum vitamins A and E, and lower activity of membrane Na + /K + -ATPase in OCD patients compared to controls. These suggest an imbalance between oxidant and antioxidant factors in OCD patients that might play a fundamental role in the etiopathogenesis of OCD., Competing Interests: The authors had no conflicts of interest., (Copyright © 2024 Amir Hossein Mohammadi et al.)