Treatment guidelines recommend the use of inhaled long-acting bronchodilators to alleviate symptoms and reduce the risk of exacerbations in patients with moderate-to-very-severe chronic obstructive pulmonary disease (COPD) but do not specify whether a long-acting anticholinergic drug or a β(2)-agonist is the preferred agent. We investigated whether the anticholinergic drug tiotropium is superior to the β(2)-agonist salmeterol in preventing exacerbations of COPD.