BACKGROUND: To characterize the clinical features and determine the prognostic factors of severe community-acquired pneumonia. This study is the first of its kind in Korea. METHODS: Recruited were 40 patients diagnosed as severe community-acquired pneumonia in Hallym University Hospital from January 1, 1989 through July 31, 1996. Patients were analysed retrospectively for age, sex, underlying disease, respiration rate, hypoxemia, requirement of mechanical ventilation, involvement on chest radiograph, shock, and the serum concentration of BUN and albumin. All parameters were compared between survived and dead group. RESULTS: Male to female ratio was 2.07 : 1. The mean age was 63.1+/-l7.5years(range 25-90years) with 65% of patients aged equal to or more than 60. The major underlying diseases were old pulmonary tuberculosis(12.5%), chronic obstructive pulmonary disease(7.5%), bronchial asthma(5%), bronchiectasis(2.5%), and diabetes mellitus(22.5%). Microbiologic diagnosis was made in 26 out of 40 patients(65%). The most common causative organism was S. Pneumoniae(17.5%, 7/40) followed by S. aureus(15.0%, 6/40), K. Pncumoniac(12.5%, 5/40), M. tubercut osis(7.5%, 3/40), H. influenzae(2.5%, 1/40), coagulase negative staphylococcus(2.5%, 1/40), P aeruginosa(2.5%, 1/40), E. cloaceae(2.5%, 1/40), and E. coli(2.5%, 1/40). M. Pneumoniae was detected in no patient. The most frequent drugs administered in single or combination therapy were aminoglycosides(75%, 30/40), second- and third-generation cephalosporin(40%, 16/40 and 27.5%, 11/40), macro]ides(27.5%, 11/40), and amoxicillin/clavulanic acid(22.5%, 9/40). Of the 40 patients, 14 died of severe community-acquired pneumonia(37.S%). Among them, seven patients (50%) expired within 72h of hospital arrival. According to multivariate analysis, mortality was significantly associated with requirement of mechanical ventilation, bilateral pulmonary involvement, and serum albumin < or = 3.0g/dl. CONCLUSION: An understanding of the clinical charateristics and prognostic factors in severe community-acquired pneumonia identified in this study will optimize therapeutic approach in this disease and help decreasing its notorious mortality rate.