A multinational, multicentre, randomized, prospective, parallel-group study compared treatment with ciprofloxacin administered as an oral suspension (500 mg twice daily for 7 days) with co-amoxiclav tablets (625 mg three times daily for 7 days) in patients suffering from acute exacerbations of chronic bronchitis (AECB). A total of 147 of 165 cases treated with ciprofloxacin (89.1%) and 146 of 162 cases treated with co-amoxiclav (90.1%) were classified as being clinical successes at the primary efficacy assessment 7 days after the end of therapy (assessed as reduced cough, improvement in dyspnoea, reduction in 24-h sputum volume or reduced purulence of sputum). Treatment equivalence was statistically confirmed; treatment difference:--1.0%, 95% CI--6.6% and 4.5%. Before treatment, 128 bacterial strains were isolated from 103 patients (60 ciprofloxacin and 68 co-amoxiclav). The most commonly isolated organism was Haemophilus influenzae (60 isolates), followed by Moraxella catarrhalis (12 isolates), Streptococcus pneumoniae (11 isolates) and Staphylococcus aureus (10 isolates). At day 14, 40 of 46 ciprofloxacin-treated patients (87.0%) and 46 of 55 co-amoxiclav-treated patients (83.6%) who were valid for bacteriological analysis were classified as being bacteriological success (classed as eradication, eradication with colonization or presumed eradication; treatment difference: 3.3%, 95% CI--8.3% and 14.9%). The adverse event profile was comparable between treatment groups. Most adverse events considered possibly or probably related to study drug were related to the gastrointestinal system and were of mild or moderate severity: nausea (13% ciprofloxacin, 10.6% co-amoxiclav), flatulence (10.3% ciprofloxacin, 3.9% co-amoxiclav), abdominal pain (7.6% ciprofloxacin, 7.3% co-amoxiclav) and diarrhoea (4.3% ciprofloxacin, 6.7% co-amoxiclav). We concluded that a 7-day course of ciprofloxacin suspension is equivalent to a 7-day course of co-amoxiclav tablets in terms of clinical and bacteriological efficacy and tolerability for the treatment of AECB. Thus, ciprofloxacin suspension may offer a suitable alternative treatment for AECB patients who have difficulty in swallowing, or who prefer liquid medications to tablets.