The effect over time of regular treatment with montelukast (MNT) in inhibiting exercise-induced bronchoconstriction (EIB) has never been evaluated in children. The aim of the present study was to examine the preventive effect of MNT against EIB in children at different time-points over a 4-week treatment period. Thirty-two asthmatic children (aged 6-12 yrs) were enrolled in a double-blinded, randomised, parallel group design to receive a 4-week treatment with MNT (5 mg chewable tablets administered once daily in the evening) or placebo. Exercise challenge was performed at baseline and after 3, 7 and 28 days of treatment, 20-24 h after dosing. MNT was significantly more protective than placebo against EIB at each time. The mean percentage drop of forced expiratory volume in one second (FEV1) was 24.6, 13.6, 12.0 and 11.6 for MNT, and 24.4, 22.4, 21.8 and 21.0 for placebo, at baseline and after 3, 7 and 28 days, respectively. For each drug, no significant difference in the percentage drop of FEV1 was found between different days. Regular treatment with montelukast provided significant protection against exercise-induced bronchoconstriction in asthmatic children over a 4-week period with no tolerance to the bronchoprotective effect.