This investigation compared the efficacy of oral formulations of amoxycillin/clavulanate and ciprofloxacin in acute exacerbations of chronic bronchitis. Forty patients were randomized to receive either Augmentin (1,000 mg amoxycillin +250 mg clavulanate) tds or ciprofloxacin (500 mg) bd. During and before therapy sputum samples were taken for bacteriology, sputum volume measurement and histamine determination. Lung function was also monitored. From sputum, 143 bacterial isolates and 15 yeast strains were obtained before therapy. During therapy with amoxycillin/clavulanate the incidence of Gram-positive isolates decreased significantly whereas ciprofloxacin left their frequency unchanged. On the other hand, ciprofloxacin more effectively diminished the incidence of Gram-negative isolates. Yeasts were grown from the sputum of several patients before and during therapy. Their number did not increase during amoxycillin/clavulanate therapy while it increased under ciprofloxacin. The groups showed no significant differences with regard to sputum production, histamine concentration or lung function. The majority of patients (28/40) acknowledged improvement of their symptoms. There was no significant difference between the groups. On the basis of these results both amoxycillin/clavulanate and ciprofloxacin appear of similar value for treatment of patients with exacerbations of chronic bronchitis.