An open-label, randomized, comparative, parallel-group study of ofloxacin and trimethoprim-sulfamethoxazole was performed in 162 outpatients diagnosed with acute exacerbation of chronic bronchitis. Ofloxacin 400 mg once daily was administered orally; the dose could be increased to 400 mg twice daily if patients had not improved after 72 hours of treatment. The other treatment group received trimethoprim-sulfamethoxazole 960 mg twice daily orally. The duration of treatment was 10 to 14 days for both treatment groups. Successful clinical response (defined as cure or major improvement in symptoms) was achieved in 65 (80.2%) patients in the ofloxacin group and in 42 (51.9%) patients in the trimethoprim-sulfamethoxazole group; the differences between groups were statistically significant. Bacteriologic success (classified as eradication or presumptive eradication of the causative pathogen) was achieved in 27 (50.0%) patients in the ofloxacin group and in 16 (29.1%) patients in the trimethoprim-sulfamethoxazole group; these differences between groups were also statistically significant. Reinfection occurred in 18 (33.3%) and 4 (7.3%) patients treated with ofloxacin and trimethoprim-sulfamethoxazole, respectively. No serious adverse events were reported.