• The magnitude of protection and the number of subjects with ≥50% protection (considered to be clinically relevant) following treatment [ Time Frame: Before and after 2 weeks of treatment in each 4 arm ] [ Designated as safety issue: No ]
  

• Change in sputum cell counts and fluid phase inflammatory mediators [ Time Frame: Before and after 2 weeks of treatment in each 4 arm ] [ Designated as safety issue: No ]
  
• Change in baseline exhaled nitric oxide (eNO) and response to exercise challenge [ Time Frame: Before and after 2 weeks of treatment in each 4 arm ] [ Designated as safety issue: No ]
  
• Change in EBC (exhaled breath condensate) inflammatory mediators from baseline [ Time Frame: Before and after 2 weeks of treatment in each 4 arm ] [ Designated as safety issue: No ]
  
• The change in exercise induced maximal percent fall in FEV1 (maxFEV1%) and area under the curve (AUC0-30) relative to pre-exercise FEV1 up to 30 minutes following exercise challenge [ Time Frame: Before and after 2 weeks of treatment in each 4 arm ] [ Designated as safety issue: No ]
  

The study will be a 2 centre randomised, double blind, placebo controlled 4-way cross over study comparing montelukast, budesonide and their combination on exercise-induced bronchoconstriction (EIB). Subjects with stable mild to moderate asthma, between the ages of 8-35 years, not on controller asthma therapy will be screened. A maxFEV1% ≥15% following a standardized dry air exercise challenge at screening (SC) and following a 1 week run-in (V-1) is required to qualify. Once qualified, subjects will return the following day (V0) for a skin prick test and a methacholine inhalation test before being randomised to 1 of 4 treatment sequences. Each treatment will be given for a total of 14 days with a 4 weeks washout. During each period, subjects will attend the laboratory on 2 occasions, at the beginning and end of each period. Baseline spirometry; eNO; EBC will be performed, followed by an exercise challenge with serial FEV1 and eNO measurements up to 30 minutes post exercise and post-exercise EBC and sputum induction at each evaluation visit. There will be a total of 10 visits and the duration of study will be approximately 148 days (21 weeks). All visits will be scheduled within 2 hours of the post-run visit (V1) and at least 8 hours after the last dose of trial medications.

Investigational Product, Dose and Administration:

Montelukast 5mg (<15 years) or 10mg (and matching placebo) will be taken 1 tablet in the evening together with inhaled budesonide turbuhalers 200ug (and matching placebo) taken 1 puff in the morning and 1 puff in the evening.

Efficacy on EIB:

The changes between pre and post treatment exercise-induced maxFEV1% and area under the curve (AUC0-30) will be compared between the 4 periods. MaxFEV1% will be calculated as pre-exercise FEV1 minus post exercise lowest FEV1 divided by pre-exercise FEV1 multiplied by 100; AUC will be calculated using the trapezoidal rule. The protection provided by the active treatments will be defined as the pre-treatment maxFEV1% minus post-treatment maxFEV1% divided by pre-treatment maxFEV1% x 100. The proportions of subjects attaining ≥50% protection will be compared between the 4 arms.

Efficacy on eNO and inflammatory parameters measured in sputum and EBC Changes in % and absolute counts of sputum differential cells, eNO and inflammatory mediators (in EBC/ sputum) at baseline and in response to exercise will be compared between the 4 arms.