The efficacy and safety of 500 mg clarithromycin and 1000 mg josamycin both given twice daily for a maximum of 14 days were compared in the treatment of 72 hospitalized patients with bacterial pneumonia. The predominant pathogens isolated were Streptococcus pneumoniae and Staphylococcus aureus. Clinical success was reported for 91.5% of patients treated with clarithromycin and for 87.0% of those treated with josamycin. Eradication of the causative pathogen was noted in 85.7% of patients receiving clarithromycin and in 90% of those receiving josamycin. Adverse events considered probably to relate to therapy were experienced by 2% of patients treated with clarithromycin and by 12.5% of those treated with josamycin; one patient treated with josamycin was withdrawn because of severe nausea and moderate vomiting. Treatment with clarithromycin at half the dosage of josamycin was found to have comparable efficacy and to be associated with a lower incidence of adverse events.