In a controlled single-blind study of 12 weeks' duration the prophylactic anti-asthmatic effects of ketotifen and clemastine in children with bronchial asthma were compared. The drugs were administered in syrup form in doses from 1 to 2 mg per day according to the body-weight. Fifty-seven children, twenty-nine in the ketotifen group and twenty-eight in the clemastine group took part. The clinical parameters, namely asthmatic complaints, dyspnoea and total duration of asthmatic attacks initially improved with both drugs but only with ketotifen was there a further marked benefit leading to a significant superiority of this drug over clemastine in the 8th and 12th week of treatment. In the overall assessment ketotifen was considered to be very effective and effective in 29%. Both drugs were well tolerated.