In a single-blind trial 29 patients with corticosteroid-dependent asthma reduced their daily dose of oral prednisolone by 1 mg/week while using a placebo inhaler until an unacceptable degree of asthma occurred. Betamethasone-17-vlaerate in a dose of 800 mug/day and if necessary 1600 mug/day was then substituted for the placebo inhaler and the reduction of oral prednisolone continued until the prednisolone was withdrawn completely or an unacceptable degree of asthma recurred. In 22 patients (76%) prednisolone was withdrawn completely, 11 on 800 mug and 11 on 1600 mug betamethasone-17-valerate. The mean reduction of prednisolone was 3-8 mg on placebo, 5-4 mg on 800 mug and a further 1-8 mg in patients requiring 1600 mug of betamethasone-17-valerate. The hypothalamic pituitary adrenal (HPA) axis was assessed by tetracosactrin and insulin stress tests at the start of the study and after withdrawal of oral prednisolone. The results indicate that an HPA axis which is completely suppressed by systemic corticosteroids can regain normal integrity when the systemic steroid is replaced by betamethasone-17-valerate in a dose of either 800 mug/day or 1600 mug/day. Candidiasis was observed, but will be reported later.