It is not proven unequivocally whether a combination of an intranasal corticosteroids (INS) and a cysteinyl leukotriene receptor antagonist has greater efficacy than INS in the treatment of severe allergic rhinitis (AR) . We performed a randomized, open-label study in 46 seasonal AR subjects receiving budesonide (BD, 256ug) plus montelukast (MNT, 10 mg) or BD alone (256ug) for 2 weeks. Visual analog scale (VAS) scores, nasal cavity volume (NCV), nasal airway resistance (NAR) and fractional exhaled nitric oxide (FeNO) were assessed before and at end of treatments as the primary treatment outcomes. Similarly, histamine, eosinophil cationic protein (ECP) and cysteinyl-leukotrienes (Cyslts) in nasal secretion and Th1/Th2 cells in nasal mucosa were evaluated as the secondary treatment outcomes.