In children, intermittent asthma is the most common pattern and is responsible for the majority of exacerbations. Montelukast has a rapid onset of action and may be effective if used intermittently. Objectives To determine whether a short course of montelukast in children with intermittent asthma would modify the severity of an asthma episode. Methods Children, aged 2-14 years with intermittent asthma participated in this multi-center, randomized, double-blind, placebo-controlled clinical trial over a 12 month period. Treatment with montelukast or placebo was initiated by parents at the onset of URTI or asthma symptoms and continued for a minimum of 7 days or until symptoms had resolved for 48 hours. Measurements and Main Results 220 children were randomised, 107 to montelukast and 113 to placebo. There were 681 treated episodes (345 montelukast, 336 placebo) provided by 202 patients. The montelukast group had 163 unscheduled healthcare resource utilisations for asthma compared to 228 in the placebo group (AR 0.65 (95% CI 0.47-0.89). There was a non-significant reduction in specialist attendances and hospitalisations, duration of episode, or -agonist and prednisolone use. Symptoms were reduced by 14% and nights awakened by 8.6% (p=0.043), days off from school or childcare by 37% and parent time off from work by 33% (p<0.0001 for both). Conclusions A short course of montelukast, introduced at the first signs of an asthma episode, results in a modest reduction in acute health care resource utilisation, symptoms, time off from school and parental time off from work in children with intermittent asthma.