An open study was carried out in 74 wheezy infants and children, average age 16 months, to assess the efficacy and tolerance of ketotifen given in the form of a syrup at a dosage of 0.02 to 0.03 mg/kg body weight twice daily over a period of 12 weeks. Treatment was assessed as being successful, by both parents and physician, in 62 (84%) of the patients. Wheeze completely disappeared in 42 (57%) patients, cough in 50 (67%) and bronchial hypersecretion in 47 (64%). Patients who developed a superimposed respiratory infection during the treatment period had milder symptoms than usual; these lasted for a shorter time and in most patients it was not necessary to give antibiotics. Forty-one of the 62 patients who had responded successfully to ketotifen suffered a relapse when treatment was withdrawn at the end of the trial period. Ketotifen was well-tolerated by the majority of patients. The side-effect most frequently reported by the parents was dry mouth, which occurred in 21 patients.