Although cysteinyl-leukotriene receptor antagonists were recently approved for use in allergic rhinitis (AR), there has been no study to date investigating their application in children. The aim was to evaluate whether montelukast provides any benefit in nasal allergen challenge-induced symptoms in children, and whether it could improve the control provided by an antihistamine during pollen season. Two randomized studies, one a double-blind, placebo-controlled, nasal allergen challenge study and one an open-label, cross-over, parallel-group clinical study, were performed in 18 (11.7 ± 0.7 years) and 32 children (10.5 ± 0.5 years), respectively, with grass pollen allergy. In the first study, the effect of a single dose of montelukast and its combination with loratadine were compared with placebo on nasal responses induced by allergen challenge. In the second study, the additive effect of montelukast to loratadine was tested in an open-label cross-over clinical study. In the challenge study, early-phase and late-phase nasal reactions peaked at 15 min and 4 h after the challenge respectively. During the early phase, combination improved total nasal symptoms (p = 0.004) during the first hour and sneezing (p = 0.012) at 15 min compared with placebo group. During the late phase, montelukast (p = 0.017) and combination (p = 0.011) caused less nasal obstruction at 4 h and combination caused less sneezing at 6 h (p = 0.015). In the clinical trial, montelukast provided protection on seasonal increase in pulmonary symptoms [0 (0, 14) vs. 6.5 (0, 27.7); p = 0.016] and on the decrease in FEF25−75 [−0.09 (−0.34, 0.17) vs. −0.28 (−0.66, 0.02); p = 0.002]. However, there was no improvement in nasal symptoms and flows. Although we showed protection against nasal challenge-induced congestion with montelukast, we were not able to show the same in the clinical study possibly because of low pollen counts and mildness of the symptoms of the patients with AR. However, montelukast provided better control of pulmonary symptoms and protection from seasonal decrease in lung function, indicating its potential therapeutic benefit in children with AR.