SUMMARYCefaclor and amoxycillin/clavulanate are active against Haemophilus influenzae, Streptococcus pneumoniae, Moraxella catarrhalis and Staphylococcus aureus ‐ pathogens commonly associated with acute exacerbations of chronic bronchitis (AECB). This randomised, parallel‐group, single‐blind, multicentre study investigated the comparative efficacy and safety of 7‐day treatment regimens of cefaclor AF (750 mg b.d. [n=73]) and amoxycillin/clavulanate (875/125 mg b.d. [n=72]) in AECB. A favourable clinical response was obtained in 95.9% of patients on cefaclor AF and 97.2% of patients on amoxycillin/clavulanate. There were no statistically significant differences between the groups for improvement in clinical response measured by pulmonary peak expiratory flow (PPEF), or for common symptoms associated with AECB. Both agents were well tolerated, with no statistically significant differences in overall safety; however, nausea and vomiting, and abdominal pain, the most frequently occurring adverse events in the amoxycillin/clavulanate group, were not reported in the cefaclor group. In conclusion, cefaclor AF and amoxycillin/clavulanate have similar efficacy and safety profiles in the treatment of AECB.