BACKGROUND Aclidinium is a novel, long-acting muscarinic antagonist indicated for maintenance treatment of COPD. METHODS In this 52-week, parallel-group, double-blind study, patients with moderate-to-severe COPD were randomized (1:1) to receive aclidinium twice-daily (BID) 200 μg or 400 μg via a novel, dry powder inhaler (Genuair(®)/Pressair(®)) [Registered trademarks of Almirall, SA, Barcelona, Spain for use within the European Union, Iceland, Norway, and Switzerland as Genuair(®) and within the United States as Pressair(®)]. Safety, the primary objective, was assessed via adverse events (AEs), clinical laboratory tests, vital signs, and 12-lead electrocardiograms. Efficacy was evaluated using spirometry, SGRQ, and rescue medication use. RESULTS A total of 605 patients were randomized in the study. The percentage of patients reporting any treatment-emergent AE (TEAE) was comparable between groups; most TEAEs were mild or moderate. Anticholinergic TEAEs were reported by low percentages of patients in either treatment group (dry mouth: 200 μg, 1.3%; 400 μg, 2.7%; constipation: 200 μg, 2.9%; 400 μg, 1.7%). Cardiac TEAEs were also reported by a low percentage of patients (<2% for any event in any group) and did not appear to be dose dependent. There were no clinically relevant abnormalities in other safety outcomes. Both aclidinium 200 μg and 400 μg resulted in improvements from baseline to Week 52 in FEV1, with numerically greater increases observed with the higher dose. Clinically important improvements in SGRQ scores and a reduction in rescue medication use were observed throughout the study for both doses. CONCLUSIONS Long-term treatment with aclidinium 200 μg or 400 μg BID was well tolerated, with sustained benefits in lung function and health status in patients with COPD throughout the 1-year study.