We compared the clinical and microbiological efficacy of dirithromycin with that of azithromycin in outpatients with acute bacterial exacerbations of chronic bronchitis who could be graded into stage III according to Ball's system of stratification. A total of 80 patients was studied. Of these, 40 were treated with dirithromycin as a once-daily dose of 500 mg for 5 days, and 40 with azithromycin as a once-daily dose of 500 mg for 3 days. At post-therapy, treatment success (cure or improvement) was achieved in 36 out of 40 (90%) patients receiving dirithromycin compared with 37 out of 40 (92.5%) in the azithromycin group. At the late post-therapy visit, 34 out of 36 (94.4%) dirithromycin-treated patients were cured as were 33 of 37 (89.2%) azithromycin-treated patients. A small proportion of patients treated with dirithromycin (10%) or with azithromycin (12.5%) suffered mild side effects. Gastrointestinal disorders, including abdominal cramps, nausea, or diarrhea, were common adverse effects. The main pathogens isolated before treatment were Streptococcus pneumoniae, Haemophilus influenzae, and Moraxella catarrhalis. Eradication rates at the end of treatment were 90% (36 out of 40) for the dirithromycin group and 92.5% (37 out of 40) for the azithromycin group. Persistence of H. influenzae isolates was found in 3 out of 11 (27.3%) patients treated with dirithromycin and in 2 out of 9 (22.2%) who had received azithromycin. At the late post-therapy visit, eradication occurred in 34 out of 36 (94.4%) strains in the dirithromycin group and in 33 out of 37 (89.2%) in the azithromycin group. We conclude that dirithromycin and azithromycin appear to be equally effective in the treatment of acute bacterial exacerbations of chronic bronchitis.