Background: Nitric oxide (NO) imbalance appears to be important in the pathogenesis of allergic rhinitis. NO is synthesized from L-arginine by NO synthase (NOS). Competing with NOS for L-arginine is arginase, which catalyzes the hydrolysis of arginine to urea and ornithine. Therefore, increased serum arginase activity could potentially limit NO production catalyzed by inducible NOS, thus contributing to allergic rhinitis. This study was designed to investigate the effect of the cysteinyl leukotriene type 1 receptor antagonist, montelukast sodium on serum arginase levels in patients with seasonal allergic rhinitis.