Background: This study compared the bronchodilator efficacy and safety of tiotropium inhalation capsules (18μg once daily) with a ipratropium metered dose inhaler (2 puffs of 20μg q.i.d.) in patients with chronic obstructive pulmonary disease (COPD). Method: After the initial screening assessment and a two week run in period, patients received either tiotropium 18μg once daily or ipratropium 40μg four times daily over a period of 4 weeks in a double blind, double dummy, parallel group study. The outcome measures were the lung function, the daily records of the peak expiratory flow rate (PEFR), the patients' questionnaire, and the use of concomitant salbutamol. The forced expiratory volume in one second (FEV1) and the forced vital capacity (FVC) were measured 5 minutes before inhalation, and 0.5, 1, 2 and 3 hours after inhaling the study drug on days 0, 14 and 28. Result: In 16 centers, 134 patients with a mean (SD) age of 66 (7) years and a predicted FEV1 of 42 (12)% were analyzed. The trough FEV1 response was significantly higher in the tiotropium group than in the ipratropium group after a four week treatment period. The weekly mean morning PEFR of the tiotropium group was consistently higher than that of the ipratropium group during the 4 week treatment period with differences ranging from 12.52 to 13.88 l/min, which were statistically significant. Tiotropium was well tolerated by the COPD patients during the 4-week treatment period and had a similar safety profile to ipratropium. Conclusion: This study shows that tiotropium administrated once daily has a superior bronchodilator effect with a similar safety profile in treating COPD patients compared with ipratropium, inhaled four times daily.