The efficacy and safety of oral 500 mg clarithromycin given twice daily and 500 mg josamycin given three times daily were compared for up to 14 days in the treatment of 103 out-patients with acute exacerbations of chronic bronchitis in an open, randomized study. The predominant pathogens isolated were Streptococcus pneumoniae, Haemophilus influenzae and Staphylococcus aureus. Clinical cures were experienced by 85% of the clarithromycin-treated patients, with an additional 10% showing improvement. In josamycin-treated patients, 76% of the patients experienced clinical cures and an additional 19% showed improvement. Eradication of the causative pathogen occurred in approximately 95% of the patients in both treatment groups. Adverse events, which all involved the gastro-intestinal tract and were mild, transient and probably related to therapy, were experienced by 5.8% and 7.8%, respectively, of the clarithromycin- and josamycin-treated patients. It is concluded that clarithromycin given twice daily is as effective as josamycin given three times daily.