We plan a randomized double blind double-dummy trial of 190 previously healthy children 2-17 years of age presenting to the Emergency Department (ED) at the Hospital for Sick Children in Toronto with mild to moderate acute asthma, with the Pulmonary Index score ≤ 11 points and PRAM ≤ 8 points. Asthma will be defined as at least the second episode of wheezing, with signs of lower airway obstruction. All participating children will receive a single dose of oral prednisolone 2mg/kg (max 60 mg) on arrival and standardized inhaled salbutamol in the ED and for five days thereafter. At discharge from ED children will be randomized to two interventional groups. Those in the Montelukast group will get oral Montelukast 4 mg (2-5 year olds), 5 mg (6-14 year olds), and 10 mg (15-17 year olds) as well as daily prednis(ol)one placebo 24 hours after the ED dose of prednisolone and at 48, 72, 96 and 120 hours, while those in the prednisolone group will receive Montelukast placebo and daily oral prednisolone 1mg/kg (max 60 mg) for five further doses at these times.