Necrotizing fasciitis is a life-threatening soft-tissue infection primarily involving the superficial fascia. This study investigated the microbiologic characteristics and determinants of mortality of this disease. The medical records of 87 consecutive patients with a diagnosis of necrotizing fasciitis from 1999 to 2004 were retrospectively reviewed. A single pathogen was identified as the infectious agent in 59 patients (67.8%), multiple pathogens were identified in 17 patients (19.6%), and no organism was identified in 11 patients (12.6%). Klebsiella pneumoniae, identified in 17 patients, was the most commonly isolated species. The most common comorbidity was diabetes mellitus (41 patients; 53.2%). Multivariate logistic regression analysis showed that more than 1 comorbidity, thrombocytopenia, anemia, more than 24 h delay from onset of symptoms to surgery and age greater than 60 were independently associated with mortality. This study found that K. pneumoniae was the most common cause of necrotizing fasciitis. Early operative debridement was independently associated with lower mortality.