Chronic bronchitis is common among adults and infectious exacerbations contribute considerably to morbidity and mortality. We aimed to compare the safety and efficacy of moxifloxacin to clarithromycin for the treatment of patients with acute bacterial exacerbations of chronic bronchitis (ABECB) using a prospective, randomized, double-blind, parallel group trial. Between November 21, 1996 and April 7, 1998, 936 patients with acute exacerbations of chronic bronchitis (AECB) were enrolled at 56 centers across the United States of which 491 (52%) had ABECB (i.e. pretherapy pathogen). Patients were randomized to either oral moxifloxacin 400 mg administer once daily, for either 5 or 10 days, or clarithromycin 500 mg bid for 10 days. For the purpose of study blinding, the patients taking moxifloxacin received placebo to maintain uniform dosing. The main outcome measures were bacteriological response at the end of therapy (post-therapy days 0-6) and follow-up (7-17 days post-therapy) visits, as well as overall clinical response, clinical response at the end of therapy and clinical response at follow-up. Two patient populations were analyzed: efficacy-valid (i.e., those with a pretherapy pathogen) and intent-to-treat (i.e., all subjects that took drug). In 420 efficacy valid patients with a pretherapy organism, overall clinical resolution was 89% for 5 days moxifloxacin vs. 91% for 10 day moxifloxacin vs. 91% for 10 day clarithromycin. Bacteriological eradication rates at the end of therapy were 94% and 95% for 5-day moxifloxacin and 10-day moxifloxacin, respectively, and 91% for the clarithromycin group. Eradication rates at follow-up were 89% and 91% for 5-day moxifloxacin and 10-day moxifloxacin respectively, and 85% for the clarithromycin group. Among 926 intent-to-treat patients (312 5-day moxifloxacin, 302 10-day moxifloxacin and 312 clarithromycin), drug-related events were reported for 26%, 30% and 35%, respectively. Moxifloxacin 400 mg once daily, as a 5 or 10 day regimen, was found to be clinically and bacteriologically equivalent to 10 day clarithromycin for the treatment of ABECB. Given its favorable safety and tolerability profile, moxifloxacin administered once daily for 5 days may be as effective and a more convenient treatment than a standard course of clarithromycin for patients with ABECB.