In a prospective, multicenter, double-blind study, the interval to clinical relapse in patients with acute bacterial exacerbations of chronic bronchitis from whom a pretherapy pathogen was isolated was compared following treatment with ciprofloxacin or cefuroxime axetil. Clinical and microbiological responses at the end of therapy were secondary efficacy variables. Outpatients randomly received either ciprofloxacin or cefuroxime axetil (500 mg twice a day for 14 days). Three hundred seven patients with acute exacerbations of chronic bronchitis were enrolled, of whom 208 had an exacerbation due to a bacterial pathogen. Clinical resolution at the end of ciprofloxacin and cefuroxime axetil therapy for patients for whom efficacy could be evaluated was 93% and 90%, respectively. Bacteriologic eradication rates were statistically higher for ciprofloxacin recipients (96% [89 of 93]) than for cefuroxime axetil recipients (82% [80 of 97]) (P < .01). The median infection-free interval was 146 days for ciprofloxacin recipients vs. 178 days for cefuroxime axetil recipients (P = .37). In conclusion, ciprofloxacin was associated with an infection-free interval and clinical response that were similar to those associated with cefuroxime axetil, but the bacteriologic eradication rate associated with ciprofloxacin was statistically significantly higher than that associated with cefuroxime axetil.