This randomized, multicentre, double-blind, double-dummy study compared the efficacy and safety of grepafloxacin and amoxycillin in acute bacterial exacerbations of chronic bronchitis (ABECB). Patients were randomized to receive grepafloxacin 400 mg or 600 mg od, or amoxycillin 500 mg tds, for 7 or 10 days. The trial recruited 656 patients, of whom 566 (86%) completed the study. Clinical success rates at the 2 week follow-up visit for the population evaluable for clinical efficacy were 82% (165/202 patients) in the grepafloxacin 400 mg group, 85% (175/206) in the grepafloxacin 600 mg group and 85% (172/203 patients) in the amoxycillin group. The 95% confidence interval confirmed the equivalence of the two grepafloxacin doses and amoxycillin, with no significant difference between the grepafloxacin groups. The microbiological success rates at follow-up showed equivalence between the grepafloxacin 400 mg and amoxycillin groups: 86% (144/168 isolates) and 83% (162/195), respectively. The grepafloxacin 600 mg group achieved a statistically significantly higher eradication rate (92%, 150/164; 95% CI 2.0%, 16.1%) than the amoxycillin group in the follow-up assessment for microbiological and clinical efficacy (evaluable population). There was no significant difference between the two grepafloxacin treatment groups (95% CI -13.3%, 0.9%; P= 0.087). All three treatment regimens successfully eradicated the pathogens most commonly isolated during the study, including Haemophilus influenzae, Moraxella catarrhalis and Streptococcus pneumoniae. Grepafloxacin had a good safety profile, comparable to that of amoxycillin, although grepafloxacin 600 mg was associated with a higher incidence of nausea, dyspepsia and taste perversion than amoxycillin. It can be concluded that grepafloxacin 400 mg or 600 mg od is as effective as amoxycillin 500 mg tds in the treatment of ABECB.