Patients with moderate persistent asthma (n = 523; mean FEV1 77.4%) not fully controlled with inhaled corticosteroids (ICS; 400-1000 microg/day) were randomized to receive either once-daily budesonide/formoterol (160/4.5 microg, two inhalations); or twice-daily budesonide/formoterol (160/4.5 microg, one inhalation); or budesonide (400 microg) once-daily for 12 weeks. Once-daily dosing was administered in the evening and twice-daily dosing was administered in the morning and evening. All patients received twice-daily budesonide (200 microg) during a 2-week run-in. Compared with budesonide alone, change in mean morning and evening peak expiratory flow was greater in the once-daily budesonide/formoterol group (27 and 171 min(-1), respectively; P < 0.001) and twice-daily budesonide/formoterol group (23 and 24 l min(-1), respectively; P < 0.001). Night awakenings, symptom-free days, reliever-use-free days and asthma-control days were all improved during once-daily budesonide/formoterol therapy vs. budesonide (P < or = 0.05). Similar improvements were also seen with twice-daily budesonide/formoterol (P < or = 0.05). The risk of a mild exacerbation was reduced after once- and twice-daily budesonide/formoterol vs. budesonide (38% and 35%, respectively; P < 0.002). All treatments were well tolerated. Budesonide/formoterol, once- or twice-daily, in a single inhaler improved asthma symptoms and exacerbations compared with budesonide. In the majority of patients with moderate persistent asthma requiring ICS and long-acting beta-agonists, once-daily formoterol/budesonide provided sustained efficacy over 24 h, similar to twice-daily dosing.