Inhaled beclomethasone dipropionate and inhaled betamethasone valerate have been compared with oral prednisone in the treatment of 75 patients with asthma who were starting long-term corticosteroids for the first time. Both of the inhaled corticosteroids controlled asthma as well as did oral prednisone in those who had responded to therapy in the initial period of the trial. A daily dose of 400 mug of inhaled drug was approximately equivalent to 7-5 mg daily of prednisone. Prednisone suppressed the adrenal response to tetracosactrin, whereas the mean responses in the groups receiving inhaled corticosteroids did not change significantly from pre-trial values. The 30% incidence of other systemic unwanted effects of prednisone contrasted sharply with the low incidence (5%) of symptomatic oropharyngeal candidiasis in the patients receiving inhaled corticosteroids. In a sample of 19 patients no change in exfoliative cytology was detected over the period of the trial nor was there any evidence of fungal colonisation of the bronchial tree. There was no difference between the three treatment groups in the number of antibiotic courses prescribed. The persistent production of sputum made no difference to the response to inhaled corticosteroids. Patients not on sodium cromoglycate did as well in the trial as those receiving sodium cromoglycate. Both inhaled beclomethasone dipropionate and inhaled betamethasone valerate have advantages over oral prednisone in the maintenance treatment of patients with asthma, but in the management of exacerbations systemic corticosteroids will usually be needed as a supplement to inhaled therapy.