The purpose of this study is to evaluate the safety, tolerability and efficacy of inhaled aclidinium bromide at two dose levels in patients with moderate to severe, stable chronic obstructive pulmonary disease. The study will be 56 weeks in duration; a 2-week rin-in period, a 52-week treatment period and a 2-week follow up phone call. All patients will be randomized to one of two doses of aclidinium bromide.