To compare the long-term (one-year) bronchodilator efficacy and safety of COMBIVENT hydrofluoroalkane (HFA) Inhalation Aerosol to COMBIVENT chlorofluorocarbon (CFC) Inhalation Aerosol and Placebo formulations of each in patients with COPD. In addition, steady state pharmacokinetics over one dosing interval following four weeks of therapy will be characterized.