To examine the efficacy and safety of the once-daily, inhaled, long-acting muscarinic antagonist/β2-agonist combination umeclidinium/vilanterol (UMEC/VI) compared with UMEC and VI monotherapies in patients with chronic obstructive pulmonary disease (COPD). In this 24-week, double-blind, placebo-controlled, parallel-group study (ClinicalTrials.gov: NCT01313650) eligible patients were randomised 3:3:3:2 to treatment with UMEC/VI 62.5/25 mcg, UMEC 62.5 mcg, VI 25 mcg or placebo administered once daily via dry powder inhaler (N = 1532; intent-to-treat population). Primary endpoint was trough forced expiratory volume in one second (FEV1) on Day 169 (23-24 h post-dose). Additional lung-function, symptomatic, and health-related quality-of-life endpoints were assessed, including 0-6 h weighted-mean FEV1, rescue salbutamol use, Transition Dyspnoea Index (TDI), Shortness Of Breath With Daily Activity (SOBDA) and St. George's Respiratory Questionnaire (SGRQ) scores. Safety evaluations included adverse events (AEs), vital signs, 12-lead/24-h Holter electrocardiography parameters and clinical laboratory/haematology measurements. All active treatments produced statistically significant improvements in trough FEV1 compared with placebo on Day 169 (0.072-0.167 L, all p < 0.001); increases with UMEC/VI 62.5/25 mcg were significantly greater than monotherapies (0.052-0.095 L, p ≤ 0.004). Improvements were observed for UMEC/VI 62.5/25 mcg vs placebo for weighted-mean FEV1 on Day 168 (0.242 L, p < 0.001), rescue salbutamol use during Weeks 1-24 (-0.8 puffs/day, p = 0.001), TDI (1.2 units, p < 0.001), SOBDA (-0.17 units, p < 0.001) and SGRQ (-5.51 units, p < 0.001) scores. No clinically-significant changes in vital signs, electrocardiography, or laboratory parameters were observed. Once-daily UMEC/VI 62.5/25 mcg was well tolerated and provided clinically-significant improvements in lung function and symptoms in patients with COPD. Copyright © 2013 Elsevier Ltd. All rights reserved.