The aim of this trial is to evaluate the efficacy and safety of 2.5 and 5 mcg tiotropium over a 24-week treatment period as compared to placebo and salmeterol (50 mcg twice daily). Tiotropium inhalation solution delivered by the Respimat® inhaler will be examined on top of maintenance treatment with inhaled corticosteroid controller medication in patients with moderate persistent asthma. Efficacy and safety will be assessed by measuring effects on lung function, effects on asthma exacerbations, effects on quality of life, effects on asthma control, effects on health care resource utilisation, and number of adverse events.