The aim of the study is to compare the effect of roflumilast on lung function, symptoms, and use of rescue medication in patients with stable asthma. Roflumilast will be administered orally either in the morning or in the evening at one dose level. The study duration consists of a baseline period (1 to 2 weeks) and a treatment period (6 weeks). The study will provide further data on safety, tolerability, and effectiveness of roflumilast.