BACKGROUND--In addition to bronchodilator and anti-inflammatory therapy, exacerbations in patients with chronic obstructive pulmonary disease (COPD) are often treated with antibiotics. Haemophilus influenzae and Moraxella (Branhamella) catarrhalis, two important respiratory pathogens, may produce beta-lactamase which makes them resistant to ampicillin. Surveillance studies conducted in various countries have shown an increasing incidence of these beta-lactamase producing bacteria. Although this may simply be a consequence of the increasing use of antibiotics, it is possible that other factors are important. A study was undertaken to investigate whether clinical factors are related to the presence of beta-lactamase forming bacteria in the sputum of patients with COPD. METHODS--One hundred patients with COPD aged over 40 years were sequentially selected from an outpatient clinic on the basis of sputum culture results. Fifty had beta-lactamase positive (beta L+) and 50 had beta-lactamase negative (beta L-) bacteria in their sputum. Patients were included only if sputum culture results yielded one pathogen. The files of these patients were investigated for possible causative factors present during the two preceding years. RESULTS--Both groups were almost identical in terms of lung function, maintenance medication, and smoking history. The total number of antibiotic courses in the beta L+ group was higher, as were individual courses of cephalosporins, tetracyclines, and macrolides. The number of patients admitted to hospital was higher in the beta L+ group, but admissions were of equal duration in both groups. Patients admitted to hospital had poorer lung function. Risk factors for beta-lactamase producing bacteria were identified by logistic regression analysis which revealed an odds ratio for one course of antibiotics of 1.15 (95% CI 1.04 to 1.28). CONCLUSIONS--An increased number of antibiotic courses is related to a higher incidence of beta-lactamase producing bacteria and more patients had hospital admissions in the beta L+ group. beta-lactamase stable antibiotics were used more frequently in the beta L+ group, probably because prescribing was adapted to the presence of beta-lactamase producing bacteria. No other differences were found between the beta L+ and beta L- groups.