Inhaled glucocorticosteroids such as budesonide have an important role in the management of asthma. Although these agents have traditionally been given twice daily, evidence is accumulating that once-daily treatment may be as effective as twice-daily administration. The efficacy of budesonide Turbuhaler (Astra, Lund, Sweden), 400 micrograms once daily in the evening or 200 micrograms twice daily, was compared in a randomized, double-blind study involving 181 patients (75 men, 106 women; mean age 30.8 years) with mild asthma [mean forced expiratory volume in 1 s (FEV1) 92.8% pred.] who had not previously been treated with inhaled glucocorticosteroids. After a 2-week run-in period, patients were randomized to either regimen and treated for 6 weeks. This was followed by two 8-week open treatment periods, during which all patients received budesonide Turbuhaler, 200 micrograms once daily during the first period and 100 micrograms once daily during the second period. The mean change in morning peak expiratory flow (PEF) during the double-blind treatment period was 16.9 l min-1 in patients receiving once-daily treatment and 17.2 l min-1 in those receiving twice-daily treatment. Similarly, there were no significant differences in evening PEF, symptom scores, bronchodilator use or spirometry data between patients receiving once- and twice-daily treatments. The improvements in morning PEF, symptom scores and bronchodilator use seen during the double-blind treatment period were maintained during the two open treatment periods. It is concluded that once-daily treatment with budesonide Turbuhaler is as effective as an initial therapy twice-daily treatment in patients with mild persistent asthma and that the initial dose can be reduced to maintenance levels (including 100 micrograms) without loss of asthma control.