Behcet's disease(BD) is a chronic multisystemic disorder which is characterized by a relapsing systemic inflammatory process. In certain inflammatory conditions such as rheumatoid arthritis, over production of nitric oxide (NO) could damage host cells and tissues, either directly and/or following reaction with other free radicals, such as superoxide anion to form species including peroxynitrite or hydroxyl radicals. Excessive superoxide radical production and impaired antioxidant mechanism in both the neutrophils and plasma of patients with BD have been reported. Our study was designed to investigate the role of NO in BD. NO is an extremely unstable molecule and rapidly converted in vivo and in vitro to nitrate (NO3-) and nitrite ( NO2-). For this reason serum NO2- and NO3- have been used as an index of NO generation. We measured serum nitrate + nitrite levels, by using an enzymatic one-step methodology based on the reduction of nitrate to nitrite by nitrate reductase from Aspergillus species, in the presence of (3-NADPH. When compared to healthy controls, serum nitrate + nitrite levels were found to be higher in active periods of BD patients (P < 0.01). It was concluded that increased NO production in patients with BD might have critical biological activities relevant to vasculitic events in the active period of disease.