Children's use of inhalation devices can give valuable information about their adherence to asthma therapy. The aim of this study was to examine treatment adherence of low dose inhaled budesonide or placebo administered via Turbuhaler twice daily in children with mild asthma participating in an asthma trial, by comparing diary registration with the number of doses remaining in the inhaler. A total of 163 children (age 7-16 yrs, 56 females, 107 males) with mild asthma (mean baseline forced expiratory volume in one second (FEV1) was 103% of predicted), were included into a double blind, randomized study. After a two-week run-in period, the children received inhaled budesonide, either 100 microg or 200 microg daily, and/or placebo for 12 weeks. All patients used daily diary cards throughout the study. Results from 161 patients were analysed. Mean compliance according to the diary was 93%, whereas estimated mean compliance when counting remaining doses in the Turbuhaler was 77%. Overuse of medication was found in 7% of the children. There was no significant difference in compliance between sex in the study group, whereas children aged < or =9 yrs had significantly better drug adherence than older children. No significant relationship was found between symptom score and compliance. In conclusion, even with optimal patient follow-up in a clinical trial, adherence to prophylactic asthma treatment is considerably lower than the patients own reports from the use of daily diary cards.