One hundred and seven chronically wheezing infants, aged 6 months to 3 years, completed a double-blind placebo controlled multicenter study. After a two-week baseline period the patients were randomized into two groups receiving twice daily either ketotifen or placebo for a period of 12 weeks. The ketotifen dosage was 0.5 mg for children younger than one year and 1 mg for the older. During the 12-week treatment period the patients from the two groups demonstrated gradual improvement of the disease severity parameters, as compared with the baseline period. The amelioration was more marked in the ketotifen treated patients and during the 0-4, and 4-8 weeks of treatment a significant decrease was achieved in the percentage of days with a cough (p < 0.04) and in the number of wheezing episodes (p = 0.02). Moreover, a 50% reduction of the days and nights with cough and in the number of wheezing episodes occurred earlier in the ketotifen than in the placebo treated patients, i.e. after 1.2 vs 4.6; 4.4 vs 8.6 and 5.2 vs 7.2 weeks, respectively. It should be stressed that the amelioration of the asthmatic morbidity in the ketotifen group was achieved with a significantly reduced need for the concomitant use of bronchodilators. The principal side effects observed were weight gain and transient sedation. It is concluded that ketotifen may be effective in the amelioration of asthma associated symptomatology and acceleration in the natural tendency for improvement in chronic wheezing infants.