We conducted a prospective, randomised, multi-center, double-blind study of 512 German patients. We focused mainly on the exacerbation-free interval after treating a current AECB with either clarithromycin 250 mg every 12 hours for 10 days or oral levofloxacin 500 mg once daily for seven days. Patients were stratified according to the severity of the COPD, the bacterial etiology and the main pathogens isolated in sputum. A one year follow-up period allowed the identification of any new acute exacerbations and the further calculation of the interval between two exacerbations.