A randomised, double-blind trial was undertaken to investigate the efficacy and tolerability of formoterol Turbuhaler in children with mild to moderate asthma. After a two-week run-in, 248 children aged 6-17 years were randomised to receive formoterol 4.5 and 9 pmicro b.i.d. or placebo for 12 weeks. Morning PEF (primary variable), was significantly improved versus placebo only in the formoterol 9 pmicro b.i.d. group (13 l/min, 95% CCI 1.9, 24.2%; p = 0 .02). Both formoterol 4.5 and 9 pmicro significantly increased the pre-bronchodilator FEV1 by 5.2-6.7% (p < 0 .05) and reduced use of daytime relief medication versus placebo (p < 0 .05). Formoterol 9 pmicro significantly reduced night-time reliever use and awakenings due to asthma versus placebo (p < 0.05). Both doses of formoterol were as well tolerated as placebo. In conclusion, formoterol 4.5 and 9 micro b.i.d. is effective and well tolerated as maintenance therapy in children with mild to moderate asthma.