This randomized, double-blind, Phase IIIb study evaluated the 24-hour bronchodilatory efficacy of aclidinium bromide versus placebo and tiotropium in patients with moderate-to-severe chronic obstructive pulmonary disease (COPD). Methods: Patients received aclidinium 400 μg twice daily (morning and evening), tiotropium 18 μg once daily (morning), or placebo for 6 weeks. The primary endpoint was change from baseline in forced expiratory volume in 1 second area under the curve for the 24-hour period post-morning dose (FEV 1 AUC 0–24) at week 6. Secondary and additional endpoints included FEV 1 AUC 12–24, COPD symptoms (EXAcerbations of chronic pulmonary disease Tool-Respiratory Symptoms [E-RS] total score and additional symptoms questionnaire), and safety. Results: Overall, 414 patients were randomized and treated (FEV 1 1.63 L [55.8% predicted]). Compared with placebo, FEV 1 AUC 0–24 and FEV 1 AUC 12–24 were significantly increased from baseline with aclidinium (Δ = 150 mL and 160 mL, respectively; p < 0.0001) and tiotropium (Δ = 140 mL and 123 mL, respectively; p < 0.0001) at week 6. Significant improvements in E-RS total scores over 6 weeks were numerically greater with aclidinium (p < 0.0001) than tiotropium (p < 0.05) versus placebo. Only aclidinium significantly reduced the severity of early-morning cough, wheeze, shortness of breath, and phlegm, and of nighttime symptoms versus placebo (p < 0.05). Adverse-event (AE) incidence (28%) was similar between treatments. Few anticholinergic AEs (<1.5%) or serious AEs (<3%) occurred in any group. Conclusions: Aclidinium provided significant 24-hour bronchodilation versus placebo from day 1 with comparable efficacy to tiotropium after 6 weeks. Improvements in COPD symptoms were consistently numerically greater with aclidinium versus tiotropium. Aclidinium was generally well tolerated.