OBJECTIVE To test the hypothesis that once-daily treatment with fluticasone propionate is as effective as twice-daily treatment in children with well controlled asthma. METHODS Multicentre, randomised, double-blind, parallel-group study. METHODS General practice, 86 UK centres. METHODS 328 children with a diagnosis of asthma and a mean age of 10 years (range 4-16 years), mean duration of asthma of 5.5 years, and a mean percentage predicted forced expiratory volume in 1 second prior to randomisation of 93% were randomised. METHODS Patients entered a 4-week, open run-in period, receiving fluticasone propionate 50mug twice daily via a Diskustrade mark inhalation device. Patients whose asthma was well controlled according to predefined criteria were randomised to receive either fluticasone propionate 100mug at night and placebo in the morning (once daily [od] group; n = 151) or fluticasone propionate 50mug twice daily (bd) [bd group; n = 177] for 8 weeks. METHODS Mean morning peak expiratory flow over the 8 weeks of treatment, recorded daily, adjusted for baseline, age, height and sex. RESULTS The 90% CI for the treatment difference in morning peak flow (bd-od) in the intention-to-treat population was -1.9 to 5.3 L/min, demonstrating clinical equivalence between bd and od administration. Fifteen patients withdrew from the study due to asthma-related problems, ten patients from the od group and five from the bd groups (p = 0.10). Both groups remained well controlled in terms of lung function, symptoms and use of relief medication throughout the study. CONCLUSIONS Once-daily treatment with fluticasone propionate 100mug at night is as effective as 50mug twice daily in children with well controlled mild-to-moderate asthma.