The aim of this study is to assess and compare efficacy and safety of BI 54903 at medium doses twice daily and high doses once daily (evening dosing) and placebo over an 12-week treatment period in asthmatic patients aged 12 to 65 years inadequately controlled on low dose inhaled corticosteroid (ICS) therapy as demonstrated by a decrease in Forced Expiratory Volume in one second (FEV1) (not less than 10 %, and equal to or less than 25%) and an Asthma Control Questionnaire (ACQ)-6 score of not less than 1.5 at time of randomisation.