This was a 52-week, double-blind, extension study in which COPD patients previously treated with twice-daily (BID) aclidinium bromide 200 μg or 400 μg during a 12-week lead-in study (ACCORD COPD I) continued the same treatment, while patients previously receiving placebo were rerandomized (1:1) to aclidinium 200 μg or 400 μg BID. The primary objective of this study was to evaluate the long-term safety and tolerability of aclidinium treatment. Efficacy outcomes included bronchodilation, health status, and rescue medication use. A total of 467 patients completed the lead-in study and 291 patients consented to participate in the extension. At study end, the percentages of patients who reported a treatment-emergent adverse event (TEAE) were similar for both treatments (200 μg, 77.4%; 400 μg, 73.7%). Incidence of anticholinergic TEAEs was low and similar for both treatments, with dry mouth reported in only 1 patient (400 μg). Cardiac TEAEs were reported by a similarly low percentage of patients (<5% for any event in any group) with no apparent dose dependence. Improvements from baseline in lung function were greatest for patients who received continuous aclidinium treatment and those who were rerandomized from placebo to aclidinium 400 μg; these improvements were generally sustained throughout the study. Health status and overall rescue medication use was improved from baseline for both treatments. The safety profile of twice-daily aclidinium and sustained improvements in lung function and health status throughout the 52-week extension study support its use as a long-term maintenance treatment for patients with COPD. (Clinical trial registration number NCT00970268).