BACKGROUND: In 1993, American Thoracic Society (ATS) recommended a guideline for the initial management of adults with community-acquired pneumonia(CAP). However, etiologic organisms and medical system in Korea seem to be different from those in Western countries. Retrospective analysis was done to evaluate the efficacy of antibiotics chosen by ATS guideline in the treatment of Korean patients with CAP admitted to a tertiary university medical center. METHODS: Hospitalized patients with CAP at Samsung Medical Center from April 1997 through March 1998 were retrospectively reviewed. Patients who fulfilled all of the following criteria were included in this study : (1) fever >or=38degrees C (2) purulent sputum (3) pulmonary infiltrates on chest X-ray. Patients were classified as : 1) ATS group ; patients whose initial antibiotics were chosen by ATS guideline 2) Non-ATS overuse group ; additional antibiotics administered more than those of ATS guideline, and 3) Non-ATS underuse group ; initial antibiotics were insufficient to ATS guideline. Response of empirical antibiotics and etiologic organisms of 3 groups were identified. RESULTS: Sixty-four patients were enrolled. Thirty-six patients were classified into ATS group, 10 patients Non-ATS overuse group, and 18 patients Non-ATS underuse group. Thirty-three patients of 36 ATS group, 9 patients of 10 Non-ATS overuse group, and 14 patients of 18 Non-ATS underuse group showed improvement by initial empirical antibiotics. There was no statistical difference in antibiotic response between 3 groups (p>0.05). S. pneumoniae (12.5%), K. pneumoniae(9.4%), and P. aeruginosa(4.7%), Mycoplasma(3.1%) were the most commonly isolated organisms. In 18 patients with severe CAP, P. aeruginosa was isolated only in 1 patient and Legionella organism not isolated. CONCLUSION: Initial empirical antibiotics chosen by ATS guideline were effective in the management of Korean patients with CAP admitted to a tertiary hospital. However well-designed large-scale prospective study is needed to identify etiologic organisms and choose an adequate initial empirical antibiotics in Korean adults patient with severe CAP.