Recently there has been increasing evidence that free oxygen species may play an important role in the pathophysiology of various neuropsychiatric disorders. The present study was performed to assess the changes in plasma nitric oxide (NO) levels in patients with obsessive-compulsive disorder (OCD) compared to age- and sex-matched normal controls. Twenty-three patients with OCD and 23 healthy volunteers were included in the study. NO values were determined in the plasma of normal healthy controls and the OCD patients. Plasma nitrate levels in OCD patients were significantly higher than those in controls and were significantly and positively correlated with Yale-Brown Obsession Compulsion Scale scores but not related to age or to the duration of illness. These findings indicated a possible role of increased NO may be relevant to the pathophysiology of OCD.