Behret's disease (BD) is an inflammatory multisystem disorder characterized by recurrent oral and genital aphthous ulcers, arthritis, uveitis, and thrombophlebitis; it can involve several organs. However, recurrent aphthous stomatitis (RAS) can be seen without a confirmed diagnosis of BD. Moreover, there is no way of predicting whether a patient with RAS will develop BD. Nitric oxide (NO) is a free radical synthesized from L-arginine by one of the family of nitric oxide synthase (NOS) enzymes. Increased production of NO during several inflammatory and infectious processes has been recently postulated. Our aim was to investigate the serum NO levels in patients with active and inactive BD and RAS. Forty-six patients with BD, 30 patients with RAS and 30 healthy controls were enrolled in the study. The patients with BD were separated into two groups: clinically active (n=24) and inactive (n=22). A blood sample was collected from all subjects in order to determine their serum NO levels. In patients with active BD, higher serum levels of NO metabolite were found in comparison with patients with inactive BD, in patients with RAS, or healthy controls (p<0.05). We also found higher serum NO metabolite levels in patients with RAS than in healthy controls (p<0.05). In patients with inactive BD, statistically significant higher levels of serum NO levels were found in comparison with the control group (p<0.05). However, we found no statistically significant difference between the patients with inactive BD and RAS, which indicated that inactive BD cannot be distinguished from RAS by serum NO levels. We conclude that serum NO levels may be an important marker for estimating the severity of BD. However, further studies are needed to confirm our findings.