Background and Design.- Lower respiratory tract infections cause great morbidity and morta¬lity in patients with chronic obstructive pulmonary disease (COPD). The aim of this study was to find out the causative organism and the effects of azit-romycine, ampicilline sulbactam (sultamycilline), ciprofloxacine and cefaclor monohydrate therapy in COPD. One hundred and sixteen patients with COPD in acute exacerbation were randomized in¬to four groups for antibiotic treatment following lung function test and sputum. Results - The most common strains were H. inf-luenzae (30.8%), S. pneumoniae (12.0%) and M. catarrhalis (7.7%). Conclusion. - H. influenza is the most common causative organism in acute infectious attacks of COPD. Azithromycine, ampicilline sulbactam (Sul¬tamycilline), ciprofloxacine and cefaclor monohyd¬rate are of value in the treatment of acute infection.