The primary objective of this study is to investigate the effect of tiotropium vs. placebo on trough FEV1 and FEV1 2 hours post inhalation after 12 weeks of treatment. For both endpoints changes from baseline will be analysed. Three strata of patients will be included (severe COPD, i.e. <35% of predicted FEV1, moderate COPD, i.e. 35% - <50% of predicted FEV1, mild COPD, i.e. 50% - 70% of predicted FEV1, according to American Thoracic Society [ATS] criteria). The study is conducted in order to find out for which endpoint tiotropium is different from placebo in which stratum.