This study evaluated the efficacy and safety of a novel asthma management strategy--budesonide/formoterol for both maintenance and symptom relief (Symbicort Single Inhaler Therapy)--compared with a higher maintenance dose of budesonide in patients with moderate to severe asthma. This was a 12-month, randomised, double-blind, parallel-group study. Symptomatic patients with asthma (n = 1890; mean age 43 years [range 11 years-80 years], mean baseline forced expiratory volume in 1 s [FEV(1)] 70% of predicted, mean inhaled corticosteroid [ICS] dose 746 microg/day) received either budesonide (160 microg, 2 inhalations twice daily) plus terbutaline 0.4 mg as needed or a daily maintenance dose of budesonide/formoterol (160/4.5 microg, 2 inhalations once daily) with additional inhalations of budesonide/formoterol 160/4.5 microg as needed. Time to first severe exacerbation (hospitalisation/emergency room [ER] treatment or systemic steroids due to asthma worsening or a fall in morning peak expiratory flow [PEF] to < or = 70% of baseline on 2 consecutive days) was the primary outcome variable. A total of 1890 patients were randomised, of whom 1563 (83%) had severe asthma. The time to first severe exacerbation was prolonged by budesonide/formoterol single inhaler therapy (p < 0.001) compared with a higher dose of budesonide. The risk of having a severe exacerbation was 39% lower with budesonide/formoterol single inhaler therapy compared with budesonide (p < 0.001). The number needed to treat to prevent one severe exacerbation per year with budesonide/formoterol compared with budesonide was 5. The budesonide/formoterol group had 45% fewer severe exacerbations requiring medical intervention per patient compared with the budesonide group (p < 0.001). Budesonide/formoterol patients had fewer hospitalisations/ER treatments (15 vs 25 events, respectively [descriptive statistics]) and fewer treatment days with systemic steroids (1776 days vs 3177 days, respectively [descriptive statistics]) compared with budesonide patients. Budesonide/formoterol single inhaler therapy patients used less as-needed medication compared with budesonide patients (0.90 vs 1.42 inhalations/day; p < 0.001). The mean daily ICS dose was lower in the budesonide/formoterol group than in the budesonide group (466 microg/day vs 640 microg/day). Over the 12-month study period, the budesonide/formoterol group achieved asthma control sufficient to not require any additional as-needed medication on 60% of days. Overall, budesonide/formoterol single inhaler therapy gave 31 more asthma control days (a night and day with no asthma symptoms and no as-needed medication use) per patient-year and 12 additional undisturbed nights per patient-year compared with a higher dose of budesonide. Both treatments were well tolerated. Budesonide/formoterol single inhaler therapy has the potential to provide a complete asthma management approach with one inhaler, demonstrating a high level of efficacy in patients with moderate to severe asthma.