The costs of i.v. erythromycin versus azithromycin (in terms of medication use and treatment of adverse effects) when these drugs were used with other antimicrobials to treat community-acquired pneumonia (CAP) were compared. The medical records of patients receiving i.v. azithromycin or erythromycin as part of combination antimicrobial therapy for the treatment of CAP at a 473-bed level 1 trauma center in Kentucky were retrospectively reviewed. Data were collected for patients treated from December 1, 1997, through March 31, 1998. Patient data collected included occurrence of phlebitis or pain at the injection site, number of line changes due to phlebitis, and culture results. Cost data collected included drug acquisition cost, pharmacy cost of drug preparation, nursing time to administer the agent, cost of drug supplies, and cost of managing complications. Three time-and-motion studies were conducted to determine technician preparation time and pharmacist verification time. The medical records of 62 patients were identified and reviewed; 50 patients were enrolled in the study (25 in the azithromycin group and 25 in the erythromycin group). The average total days of therapy was 5.1 for the azithromycin group and 5.6 for the erythromycin group. The average total cost, including the cost of complications ($4.36 per patient in the erythromycin group), was $66.46 in the azithromycin group and $96.56 in the erythromycin group. The difference in costs between the two groups was not significant. There was no significant cost difference between azithromycin- and erythromycin-containing combination antimicrobial therapy in the treatment of CAP.