Background: Severe sepsis is a major cause of morbidity and mortality in intensive care units. This study aimed to evaluate the prevalence, characteristics, outcomes and prognostic factors of severe sepsis in a medical intensive care unit (MICU) of a tertiary care hospital in Korea. Methods: We retrospectively reviewed the medical chart of 249 patients who were admitted to a medical intensive care unit with severe sepsis. Results: From January 2000 to December 2001, 3410 patients were admitted to the ICU. The prevalence of severe sepsis was 7.3%. The mortality of severe sepsis was 64.6%. The prognostic factors for severe sepsis were the number of organ systems that acutely failed (p = 0.036) and an admission route from general wards (p = 0.018). There was no difference in the outcome of severe sepsis according to infectious organisms (p = 0.24) and the site of infections (p = 0.38). Conclusions: Severe sepsis in the MICU is a common, expensive and often fatal condition. We expect that early rescucitation and recovery from acute organ system failure will improve the outcome of severe sepsis.