It is customary to tail off the dose of oral steroids after treatment of an acute exacerbation of asthma; the main reason for this practice is to avoid rebound asthma. We have carried out a randomised double-blind study to find out whether a tapering course of oral prednisolone has any advantage over an abruptly terminated course of prednisolone for an episode of acute asthma requiring hospital admission. We studied 35 patients admitted to hospital with acute asthma; their mean peak expiratory flow rate (PEFR) on admission was 173 L/min and their mean age was 32 years (range 18-55); all were using inhaled steroids on discharge (mean dose 908 micrograms daily). Each patient received 40 mg enteric-coated prednisolone daily for 10 days followed by a tapering course of either prednisolone 5 mg tablets (active taper) or identical placebo tablets (placebo taper), reducing from 7 tablets on day 11 to no tablets by day 18. The primary outcome measure was the PEFR on waking. Both groups responded well to treatment by day 10 (mean morning PEFR: active taper group 396 L/min, placebo taper group 391 L/min). There was no further significant change in PEFR in either group during the 7 days of active or placebo tapering or during the following 10 days (repeated measures analysis of variance, active vs placebo, p = 0.82). The groups were also similar in terms of secondary outcome measures--symptom scores, PEFR after morning bronchodilator treatment, evening PEFR, and treatment failures. This study suggests that steroid tapering is unnecessary in acute asthma; a personal asthma management plan with a reserve course of prednisolone may be more appropriate.