Current guidelines recommend adding a long-acting inhaled b 2 -agonist (LABA) to inhaled corticosteroids (ICS) in patients with uncontrolled asthma. This study evaluated the novel, once-daily LABA vilanterol trifenatate (VI) in asthma patients who remained symptomatic despite existing ICS therapy.The study involved a randomised, double-blind, placebo-controlled trial of VI (3, 6.25, 12.5, 25 and 50 mg), administered once daily in the evening by dry powder inhaler for 28 days, in asthma patients aged o12 yrs symptomatic on current ICS therapy. The primary end-point was trough (24 h post-dose) forced expiratory volume in 1 s (FEV1); secondary end-points were weighted mean FEV1, peak expiratory flow (PEF), symptom-/rescue-free 24-h periods, and safety.A significant relationship was observed between VI dose and improvements in trough FEV1 (p50.037). Statistically significant increases in mean trough FEV1, relative to placebo, were documented for VI 12.5-50 mg (121-162 mL; pf0.016). Dose-related effects of VI were observed on weighted mean (0-24 h) FEV1, morning/evening PEF, and symptom-/rescue-free 24-h periods. All doses of VI were well tolerated with low incidences of recognised LABA-related adverse events (tremor 0-2%; palpitations 0-2%; glucose effects 0-1%; potassium effects 0-,1%).Once-daily VI 12.5-50 mg resulted in prolonged bronchodilation of at least 24 h with good tolerability in asthma patients receiving ICS. Based on the overall efficacy and adverse event profile from this study, the optimum dose of VI appears to be 25 mg.