Eleven children with severe perennial asthma and a poor clinical response to disodium cromoglycate were studied in a 4-month, double blind trial involving 1 month's treatment with placebo, disodium cromoglycate, betamethasone 17 valerate, and both drugs combined according to a predetermined random design. Each drug apparently reduced symptoms and improved daily peak flow and fortnightly spirometric measurements compared with placebo, but the improvement after betamethasone 17 valerate was greater in part because of the way this group of patients was selected. There did not appear to be any additional benefit when both drugs were used together. Betamethasone 17 valerate was not compared with beclomethasone diproprionate aerosol and there is no reason to believe it may be superior. Corticosteroid aerosols have potential hazards which may prove to equal those of long-term systemic steroids, but they offer the prospect of satisfactory control of symptoms without the risk of growth suppression in children with severe asthma. Disodium cromoglycate remains the drug of choice in severe childhood asthma and the use of corticosteroid aerosols should be confined to those children who fail to respond satisfactorily to disodium cromoglycate.