We have compared the effects of three-month periods of treatment with an inhaled corticosteroid, budesonide 600 micrograms twice daily and with placebo on bronchial responses to inhaled histamine and to bronchodilators in a double-blind crossover trial in 14 middle-aged male smokers (mean age, 59.6 years) with mild airways obstruction (mean FEV1 2.42 L, 80 percent predicted [range, 48 to 110 percent]). Responsiveness to inhaled histamine was assessed monthly by the provocative concentration (mg/ml) reducing FEV1 by 20 percent (PC20). Bronchodilator response to a combination of inhaled salbutamol (5 mg) and ipratropium (0.5 mg) was assessed before and after three months' treatment. Compliance with treatment was checked by weighing aerosol canisters, and by measuring plasma budesonide and metabolites. There was no significant change in FEV1 (budesonide mean 2.38 L [SEM 0.17] vs placebo 2.40 L [0.17]), vital capacity (budesonide mean 3.69 L [0.17] vs placebo 3.81 L [0.17]) or in bronchodilator responsiveness (mean increase over baseline FEV1, budesonide 11.6 [2.7] percent vs placebo 10.5 [3.2] percent). There was a small overall reduction in bronchoconstrictor responsiveness over the period of the trial, but there was no effect of 12 weeks of budesonide treatment compared with 12 weeks of placebo treatment (mean log PC20 during budesonide 0.595 [SEM 0.063], placebo 0.591 [SEM 0.055]). Following the three-month crossover trial, six men continued for nine more months to receive budesonide in a single-blind trial and the results were compared with those in six men who took no active treatment for the subsequent nine months. No improvements in baseline spirometry, home peak flow measurements, bronchoconstrictor or bronchodilator responsiveness were observed after 12 months of budesonide treatment. Thus, a regimen of budesonide treatment that consistently attenuates bronchial responsiveness in asthmatic subjects had no effect in these men; larger and longer trials will be required to establish whether a subgroup of smokers shows a favorable response.