OBJECTIVE: The authors determined the risk factors of mortality in patients with necrotizing soft-tissue infections (NSTIs) and examined the incidence and mortality from NSTI secondary to Streptococcus pyogenes. METHODS: All patients with NSTIs who were treated between January 1989 and June 1994 were analyzed for presentation, etiology, factors important in pathogenesis and treatment, and mortality. RESULTS: Sixty-five patients were identified with NSTIs secondary to postoperative wound complications (18), trauma (15), cutaneous disease (15), idiopathic causes (10), perirectal abscesses (3), strangulated hernias (2), and subcutaneous injections (2). Necrotizing soft-tissue infections were polymicrobial in 45 patients (69%). S. pyogenes was isolated in only 17% of the NSTIs, but accounted for 53% of monomicrobial infections. Eight of ten idiopathic infections were caused by a single bacterium (p = 0.0005), whereas 82% of postoperative infections were polymicrobial. An average of 3.3 operative debridements per patient and amputation in 12 patients were necessary to control infection. The overall mortality was 29%; mortality from S. pyogenes infection was only 18%. The average time from admission to operation was 90 hours in nonsurvivors versus 25 hours in survivors (p = 0.0002). Other risk factors previously associated with the development of NSTIs did not affect mortality. CONCLUSIONS: Early debridement of NSTI was associated with a significant decrease in mortality. S. pyogenes infection was the most common cause of monomicrobial NSTI, but was not associated with an increased mortality.