We investigated the possibility that corticosteroids can reduce bronchial responsiveness to histamine by mechanisms other than change in airway caliber. Ten adult asthmatic subjects were selected because their symptoms were controlled by bronchodilators alone, they had no recent respiratory infection or allergen exposure that might have temporarily altered responsiveness, their spirometry was more than 70% predicted, and they had a range of histamine bronchial hyperresponsiveness. They received, in random order and double-blind manner, beclomethasone dipropionate (400 micrograms daily) or placebo for 4 wk and then, after a 2-week washout period, they crossed over to the other treatment for 4 wk. Treatment with beclomethasone induced a small but significant reduction in bronchial responsiveness to histamine (p = 0.014). Although the improvement was too small to be considered of clinical significance, its importance lies in the mechanisms by which it was produced. Part of the improvement was related to improvement in airway caliber, but, even when the data was adjusted for this, there was still a significant difference between beclomethasone and placebo treatment (p = 0.018). The results suggest that regular treatment with corticosteroids can alter bronchial responsiveness through mechanisms other than change in airway caliber.