The aim of this trial is to evaluate the safety and efficacy of 2.5 and 5 µg tiotropium over a 52-week treatment period as compared to placebo. 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 to severe persistent asthma. Efficacy and safety will be assessed by measuring effects on lung function, effects on asthma exacerbations, effects on asthma control, and number of adverse events.