Thirty children (aged 1 to 3 years) with mild to moderate asthma were entered into a 28-week double-blind, crossover study comparing the prophylactic effect of ketotifen and placebo. A patient daily diary card was used to document symptoms and concomitant medications taken during a 2-week baseline and subsequent 12-week drug/placebo period. This was followed by a 2-week washout and 12 more weeks of drug/placebo in a crossover design. Physician assessment was performed at the beginning, at the end of each period, and at 4-week intervals throughout the drug study. No significant difference was observed between ketotifen treatment and placebo treatment in any of the study parameters that were tested. A decrease in concomitant medication usage during the first treatment period with ketotifen was observed. The principle side effect of ketotifen therapy observed in this age group was weight gain. In contrast to studies in adults, no sedation was noted.