Twenty-nine children with an average age of 10.5 years were studied with respect to the protective effect of ketotifen on pollen-induced bronchial asthma. All of them were sensitive to deciduous tree pollen, the diagnosis being verified by bronchial challenge. The children were stratified in matched pairs and randomly allocated to two treatment groups, placebo or ketotifen 1 mg twice daily. The study was double-blind and was performed during the whole of the deciduous tree pollen season. Daily pollen counts were made, allergic symptoms were noted, additional medication was given and the expiratory pulmonary flow was recorded. During the birch pollen peak the ketotifen group showed significantly fewer, and less severe asthma and rhinoconjunctivitis than the placebo group. The anti-asthmatic medication was also used significantly less than in the placebo group. Ketotifen appeared to have good protective properties in the treatment of pollen-induced asthma and rhinoconjunctivitis.