Objective. To determine the effect of montelukast on asthma during the allergy season in children with persistent asthma and seasonal aeroallergen sensitivity. Design. This 3-week double-blind, placebo-controlled, parallel-group multicenter study compared daily montelukast 5 mg chewable tablets and placebo in patients 6-14 years of age with forced expiratory volume in 1 second (FEV₁) ≥ 60 and ≤ 85% predicted, persistent asthma that is also active during allergy season, and documented sensitivity to seasonal allergens. Concomitant inhaled corticosteroid use was permitted in up to 40% of enrolled patients. The primary endpoint was the percentage change from baseline in FEV₁ over 3 weeks of treatment. Additional endpoints included the percentage change from baseline in β -agonist use, average changes in daytime and nighttime symptom score, AM and PM peak expiratory flow rate (PEFR), investigator's global asthma evaluation, and parent/guardian global asthma evaluation at the end of the treatment period. Adverse experiences (AEs) were collected to assess safety and tolerability. Results. A total of 421 patients were randomized to montelukast (N = 203) or placebo (N = 218). For the primary endpoint, the percentage change from baseline FEV₁, montelukast was not significantly different from placebo (least squares mean 9.53% vs. 9.15%, respectively; p = 0.810). Compared with placebo, montelukast was associated with significantly lower (better) investigator's global asthma evaluation (LS mean 2.71 vs. 2.98; p < 0.05) and parent/guardian global asthma evaluation (LS mean: 2.63 vs. 2.90; p < 0.05) scores. There were no significant differences between treatment groups for the other efficacy evaluations. Both treatments were well tolerated, with no significant differences observed in AE rates. Conclusion. Montelukast did not significantly improve FEV₁ compared with placebo over three weeks of treatment during the allergy season in pediatric patients with seasonal allergen sensitivity. (ClinicalTrials.gov identifier: NCT00289874)