Background: Combination long-acting bronchodilator therapy may be more effective than long-acting bronchodilator monotherapy in chronic obstructive pulmonary disease (COPD)., Objectives: To compare the efficacy and safety of once-daily umeclidinium/vilanterol (UMEC/VI) 125/25 mcg with placebo and UMEC or VI monotherapy in COPD., Methods: This was a double-blind, placebo-controlled, parallel-group study. A total of 1493 patients were randomized (3:3:3:2) to 24 weeks of treatment with UMEC/VI 125/25 mcg, UMEC 125 mcg, VI 25 mcg, or placebo once-daily via dry powder inhaler., Results: Primary efficacy 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 also assessed. Safety evaluations included: adverse events, vital signs, electrocardiography and clinical laboratory measurements. All active treatments significantly improved trough FEV1 vs placebo (0.124-0.238 L, all p<0.001). Improvements with UMEC/VI 125/25 mcg were significantly greater than for UMEC 125 mcg or VI 25 mcg (0.079 L and 0.114 L; both p≤0.001). Improvements for UMEC/VI 125/25 mcg vs placebo were observed for the transition dyspnea index (1.0 unit; p<0.001), rescue albuterol use at Weeks 1-24 (-1.5 puffs/day) and St. George's Respiratory Questionnaire (-3.60 units, p<0.001). No safety signals were observed., Conclusions: Once-daily UMEC/VI 125/25 mcg was well tolerated and provided greater improvements in lung function, health status, and dyspnea scores compared with monotherapy components and placebo over 24 weeks. This study supports the use of UMEC/VI 125/25 mcg for the maintenance treatment of COPD., Clinical Trial Registration: protocol number: DB2113361; ClinicalTrials.gov identifier: NCT01313637.