BACKGROUND--Inhaled steroids are widely used in the treatment of mild to moderate asthma. However, long term compliance with inhaled steroids is poor and administration of a single daily dose may improve compliance. METHODS--A double blind, randomised study was performed to determine whether inhaled steroids given once daily at bedtime are as efficacious as a twice daily regimen in the long term maintenance of moderate asthmatic patients. Forty adults of mean age 37 years with moderate asthma (mean (SE) forced expiratory volume in one second (FEV1) 73.6 (1.4)% predicted, mean morning peak expiratory flow (PEF) 328 l/min) were randomised to receive either a twice daily dose (400 micrograms morning and bedtime) of inhaled budesonide (group A) or a once daily dose of 800 micrograms (group B) and were followed for a period of 12 months. Asthma symptom scores (assessed according to a modified Borg scale), inhaled beta 2 agonist consumption, and peak expiratory flow rates were recorded daily. Spirometry and airways responsiveness to methacholine (PC20) were measured at the end of each period of three months of treatment. RESULTS--There was no difference between the two groups at baseline and during the follow up period in the PC20 for methacholine. However, a difference was seen between the two groups in the mean daily number of beta 2 agonist inhalations (1.4 (0.1) puffs/patient/day in group A v 2.3 (0.1) in group B), the PEF variability (episodes of decrease in PEF of > 20%) (0.22 (0.01) episodes/patient/day in group A v 0.40 (0.02) in group B), and for asthma symptom scores (0.30 (0.04) in group A v 0.42 (0.06) in group B) for the 12 month period of the study. CONCLUSIONS--Although both regimens provide good clinical control, twice daily doses of 400 micrograms inhaled budesonide are more effective than a single dose of 800 micrograms at bedtime in the long term control of stable moderate asthma.