Sixty-four patients with mild or moderate extrinsic asthma were treated with placebo for i month and thereafter with ketotifen (1 mg twice daily, orally), disodium cromoglycate (inhalation of 20 mg, four times daily), or placebo for 2 subsequent months. The trial was performed at four different centres and the treatments were compared using double-blind technique. We found no difference between the effect of ketotifen, disodium cromoglycate and placebo on the patients' daily measurements of evening peak expiratory flow, daily score values for respiratory symptoms or the number of salbutamol puffs required to control symptoms. There was no difference between the treatment groups with regard to the patients' estimates of changes in airway sensitivity to different non-specific stimuli: fumes, tobacco smoke, cold air, and exercise. The only significant effect of DSCG was a minor (4%) increase in the mean morning value for peak expiratory flow. The findings suggest that the addition of ketotifen or disodium cromoglycate to the regimen is unlikely to give further benefit in asthmatic patients, whose symptoms are reasonably well controlled by small doses of bronchodilating drugs.