ABSTRACT Aclidinium bromide is a novel, long-acting, inhaled muscarinic antagonist in development for the treatment of chronic obstructive pulmonary disease (COPD). The aim of this study was to assess the rate of onset of bronchodilation with aclidinium compared with placebo and tiotropium. This was a double-blind, double-dummy, multicenter, crossover study in COPD patients with a post-bronchodilator forced expiratory volume in 1 second (FEV(1)) ≥30% and <60% predicted. On study days, patients received single doses of aclidinium 200 μg, tiotropium 18 μg, or placebo. Serial spirometry was conducted from 10 minutes to 3 hours post-dose. The primary variable was the percentage of patients with an increase in FEV(1) of ≥10% above baseline at 30 minutes post-dose. Other assessments included change from baseline in FEV(1) and dyspnea over 3 hours post-dose. A total of 115 patients entered the study. Significantly more patients had an increase in FEV(1) of ≥10% above baseline at 30 minutes with aclidinium and tiotropium versus placebo (49.5% and 51.8% versus 13.8%; p < 0.0001). At 30 minutes, the relative increase from baseline in FEV(1) was significantly higher for aclidinium and tiotropium versus placebo (12% and 11% versus 3%; p < 0.0001). Aclidinium and tiotropium also significantly increased FEV(1) (p < 0.01) and improved the perception of dyspnea compared with placebo at all measured time points from 10 minutes to 3 hours post-dose. In conclusion, aclidinium provided effective bronchodilation, similar to that seen with tiotropium, with significant improvements compared with placebo observed from 10 minutes post-dose.