In a multicentre, double-blind, randomized, parallel study, 426 asthmatic children aged 5-15 years old received salmeterol 50 micrograms b.i.d. or placebo b.i.d. via the Diskhaler. All patients had access to inhaled salbutamol to be used on an 'as required' (p.r.n.) basis for symptomatic relief. The study design comprised a 2-week baseline, a 12-month treatment period incorporating a 2-week 'off treatment' after 6 months, and a 2-week follow-up period at the end of the trial. At the end of 12 months of treatment with salmeterol, the adjusted change from baseline for morning and evening peak expiratory flow rate (PEF) was 56 and 47 l min-1, respectively, and this was significantly greater than placebo (P < 0.01; P < 0.05). Exacerbation rates did not differ between groups and results were not dependent upon concurrent inhaled steroid use. Neither treatment caused a change of > or = 1 doubling dose in PC20/PD20 either during or on stopping treatment. Treatment with regular salmeterol 50 micrograms b.i.d. over a 12-month treatment period provides a significant, rapid and well-maintained improvement in lung function without increasing bronchial reactivity or asthma exacerbation rates compared to p.r.n. salbutamol.