Necrotizing fasciitis and myositis are life-threatening infections involving the superficial fascia and musculature, respectively. Outcome depends on early diagnosis and aggressive treatment. Here, we aimed to determine prognostic factors for necrotizing soft tissue infections.The medical records of 16 consecutive patients diagnosed with necrotizing fasciitis (n = 13) and necrotizing myositis (n = 3) from 1999 to 2004 were retrospectively reviewed. Overall survival was 81.3% for necrotizing soft tissue infections, 84.6% for necrotizing fasciitis, and 66.7% for necrotizing myositis. Injection drug use was the most common cause of infection (31.3%). Frequent comorbidities were diabetes mellitus and hepatitis B and C (25.0%). As infectious agents, group A streptococci (GAS) were identified in 10 patients and multiple pathogens in 6 patients. Lethal outcome was always associated with GAS infection and streptococcal toxic shock syndrome (STSS). In our patients, myonecrosis, GAS infection, and STSS appeared to be negative prognostic factors for survival in necrotizing soft tissue infections.