Introduction and Objectives The Phase III ATTAIN study investigated the effect of two twice daily doses of aclidinium bromide, a second-generation, long-acting muscarinic antagonist with low systemic activity, in patients with moderate to severe chronic obstructive pulmonary disease (COPD). Methods In this 24-week, double-blind study, patients were randomised (1:1:1) to receive aclidinium (200 μg, 400 μg) or placebo, twice daily. The primary endpoint was change from baseline in trough forced expiratory volume in 1 second (FEV 1 ) at Week 24. Other study assessments at 24 weeks included: change from baseline in peak FEV 1 ; percentage of patients achieving a clinically meaningful improvement in St George9s Respiratory Questionnaire total score and Transition Dyspnoea Index; COPD symptoms as assessed by the EXACT Respiratory Symptoms score; exacerbation rate based on two definitions (healthcare resource utilisation and EXAcerbations of Chronic pulmonary disease Tool). Adverse events (AEs), clinical laboratory measures, vital signs and ECGs were also assessed. Results A total of 819 patients were included in intention-to-treat (ITT) and safety populations. At Week 24, aclidinium 200 μg and 400 μg significantly improved trough FEV 1 from baseline compared with placebo (by 99 ml and 128 ml, respectively; both p Conclusion Aclidinium 200 μg and 400 μg twice daily provided clinically meaningful improvements in bronchodilation, health status, symptoms, breathlessness and exacerbation rate. Aclidinium was well tolerated with a similar safety profile for both doses; the incidence of AEs was similar to placebo.