Background: The origin of Behçet's disease (BD) is unclear. One of the prominent features of BD is vasculitis and thrombosis as a result of endothelial dysfunction. Because nitric oxide (NO) is synthesized by endothelium, we considered it as an interesting target of investigation in BD. Objective: Our purpose was to define the level of NO in the serum of patients with BD and its relation with disease activity. Methods: As an indicator for NO, serum total nitrite levels (end product of NO) were measured by Griess reaction in 52 consecutive patients with BD and compared with 32 age- and sex-matched healthy staff volunteers. Serum NO levels of active and inactive patients were compared. Results: The overall serum nitrite levels in patients with BD (40.25 ± 10.51 μmol/L) were significantly higher (P < .001) than those in healthy volunteers (25.09 ± 5.33 μmol/L). The difference in serum total nitrite levels among patients with active BD (46.74 ± 10.62 μmol/L) and inactive BD (33.24 ± 3.73 μmol/L) was significant (P < .001). Significant differences were observed in serum nitrite levels between patients in the inactive period of BD and the control group (P < .001). Conclusion: Increased NO production might be responsible for the overall inflammatory process of BD. NO seems to be related to disease activity. (J Am Acad Dermatol 2002;46:50-4.)