 This will be a prospective, double-blind, randomized, placebo-controlled, parallel-group study of the effects of montelukast on the duration of wheezing in children 12 months to 3 years of age who are under the care of a physician for a wheezing illness. Study treatment will be given for 56 days. Participants who are 2 to 3 years old will receive either 5-mg montelukast tablets or matching placebo. Participants who are 12 months to 2 years old will receive 4-mg montelukast granules or matching placebo. The primary outcome parameter of this study will be the number of days that the infant is observed to be free of wheezing by the primary caregiver. The secondary outcome parameters will be the number of wheezing episodes during the treatment period and the rate of recurrent wheezing during the follow-up period. The study, which is a consortium arrangement between the Brigham and Womens Hospital and the Hacettepe University Medical Center in Turkey, will recruit children only from the Ankara area of Hacettepe University Medical Center in Turkey. 