In this prospective, multicenter, double-blind study, the efficacy of ciprofloxacin was compared with that of clarithromycin as therapy for patients with acute bacterial exacerbations of chronic bronchitis (ABECB) from whom a pretherapy pathogen was isolated; the efficacy was measured by the infection-free interval. Clinical and microbiological responses at the end of therapy were secondary efficacy variables. Patients randomly received either ciprofloxacin or clarithromycin (500 mg twice a day for 14 days). Three hundred seventy-six patients with acute exacerbations of chronic bronchitis were enrolled in the study of whom 234 had an ABECB. Clinical resolution was observed in 90% (89 of 99) of ciprofloxacin recipients and 82% (75 of 91) of clarithromycin recipients for whom efficacy could be evaluated. The median infection-free interval was 142 days for ciprofloxacin recipients and 51 days for clarithromycin recipients (P = .15). Bacteriologic eradication rates were 91% (86 of 95) for ciprofloxacin recipients and 77% (67 of 87) for clarithromycin recipients (P = .01). In summary, compared with clarithromycin, treatment of ABECB with ciprofloxacin was associated with a trend toward a longer infection-free interval and a statistically significantly higher bacteriologic eradication rate.