This study will enroll children between 12 and 53 months of age who have experienced episodes of wheezing or coughing in the year before study entry, with at least one episode that required one of the following: oral steroids, an urgent unscheduled medical visit, an emergency room visit, or hospitalization. This study will begin with a 2-week evaluation period during which potential participants will receive placebo once a day. Parents will document their child's asthma symptoms and medication use in a daily diary. Next, at a baseline study visit, eligible participants will be randomly assigned to one of the following two 12-month treatment groups: