Current British guidelines for the administration of beta2-agonists in acute severe asthma recommend regular nebulized therapy in hospitalized patients, followed by as-required (p.r.n.) use via hand-held devices after discharge. Since beta2-agonists do not possess anti-inflammatory activity in vivo, and are thus unlikely to influence the rate of recovery from an asthma exacerbation, it was hypothesized that patients given the short-acting beta2-agonist salbutamol on an as-required basis after admission to hospital would recover as quickly as those on regular treatment, but with potential reductions in the total dose delivered. Forty-six patients with acute severe asthma were randomly assigned to either regular prescriptions of nebulized salbutamol or to usage on a p.r.n. basis, from 24 h after hospital admission. The primary outcome measures were length of hospital stay, time to recovery, and frequency of salbutamol nebulization from 24 h after admission to discharge. Secondary outcome measures were treatment side-effects (tremor, palpitations), and patient satisfaction. Length of hospital stay was reduced in those patients allocated to p.r.n. salbutamol (geometric mean (GM) 3.7 days) versus regular salbutamol (GM 4.7 days). Time taken for peak expiratory flow to reach 75% of recent best was the same in both groups. There was a highly significant reduction in the number of times nebulized therapy was delivered to the p.r.n. group (GM 7.0, range 1-30) compared with the regular treatment group (GM 14.0, range 4-57; p=0.003; 95% confidence interval for ratio of GMs 1.29-3.09). In addition, patients reported less tremor (p=0.062) and fewer palpitations (p=0.049) in the p.r.n. group. Of the patients in the p.r.n. group who had received regular nebulized therapy on previous admissions (n=12), all preferred the p.r.n. regimen. Prescribing beta2-agonists on a p.r.n. basis from 24 h after hospital admission is associated with reduced amount of drug delivered, incidence of side-effects, and possibly length of hospital stay. This has implications for the efficient use of healthcare resources.