A prospective, population-based, surveillance study of invasive soft-tissue infections due to group A streptococci was conducted in Ontario, Canada, from 1992 through 1996. Demographic and clinical information was obtained by patient interview and chart review. Isolates were characterized by M protein and T agglutination typing. The incidence of necrotizing fasciitis (NF) increased from 0.08 cases per 100,000 population in 1992 to 0.49 cases per 100,000 population in 1995. The case-fatality

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... was 13% (68 of 520 patients died). Hypotension and multiorgan dysfunction complicated 64 cases (12%), and NF complicated 119 cases (23%). Underlying diabetes, alcohol abuse, cancer, and cardiac and pulmonary disease increased the risk of disease. Prior use of nonsteroidal anti-inflammatory agents did not influence disease severity. All 197 patients without NF, underlying illness, and hypotension at presentation survived, as did 95 (99%) of 96 normotensive patients who were !65 years old but who had underlying chronic illness. Previously healthy patients without hypotension or NF may be considered for outpatient treatment. After 150 years of decreasing rates of morbidity and mortality related to group A streptococcal (GAS) infections, the past decade has witnessed a resurgence of severe disease that has not yet been explained [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] . The failure to explain this phenomenon is in part because much of what has been reported is derived from case reports and hospital-based case series that have been limited by sample size and referral biases [1, 3, 5, 8] . The objectives of this study were to prospectively obtain population-based data on risk factors for invasive GAS infection with a soft-tissue focus, to describe