Background: Cysteinyl leukotrienes have been shown to play an important role in the pathogenesis of asthma. The effect of the leukotriene receptor antagonist, montelukast, on bronchial hyperreactivity (BHR) as measured by the methacholine challenge test in school children has not been reported.Objective: To determine the effect of montelukast (Singulair®) on BHR measured by methacholine challenge and lung function tests in Thai asthmatic children aged 6-13 years.Materials and methods: This was a randomized, double-blind, placebo-controlled, crossover study performed in 29 mild to moderate persistent asthmatic children aged 6-13 years. Each child received crossover treatment with 6 weeks of montelukast (5 mg/day) and 6 weeks of placebo separated by a two-week washout period.Results: The improvement of FEV1 and FEV1/FVC after 6 weeks of treatment was significantly higher in montelukast group compared to those of placebo group (p < 0.05).After 6 weeks of treatment, mean PC20 (± SEM) in the placebo group (5.7 ± 1.41 mg/ml) was lower than in montelukast group (6.8 ± 1.74 mg/ml) but there was no significant difference (p= 0.79).Conclusion: Montelukast significantly improved FEV1 and FEV1/FVC but not BHR in mild to moderate persistent asthmatic children aged 6-13 years after the 6 weeks of treatment.