The usefulness of ketotifen for reducing bronchial hypperreactivity, acting as a prophylactic drug, was studied in children with extrinsic asthma. In an open-label phase (phase A), 2.267 asthmatic children received ketotifen orally at a dosage of 1 mg twice daily for three months. Another study, double-blind, placebo-controlled (phase B), enrolled 75 children (38 ketotifen; 37 placebo); they received the same dosage of ketotifen or placebo for five months, after a one-month placebo baseline control period. A progressive improvement in the severity of asthmatic attacks was seen throughout the three-month period (phase A). At the end of the trial, the percentage of patients with moderate or intense attacks was reduced from 77% at baseline to 18%. Significant reductions were noted in the duration and frequency of asthma attacks at each monthly evaluation. The need for the use of symptomatic medication declined significantly during the 3-month trial. In the five-month, double-blind, placebo--controlled study (phase B), significant reduction in the frequency of dyspnea was noted at the end of the trial; in the placebo group, this frequency increased. This clinical observation was confirmed by the mean consumption of sympathomatic drugs during the five-month treatment period: it declined in the ketotifen--treated patients but increased in those who received placebo. The low incidence side effects attests to the safety of ketotifen in children. Our results demonstrate that ketotifen is both safe and effective for prophylactic use in asthmatic children.