During the past few years, there has been an apparent increase in serious infections due to group A streptococci (GAS) worldwide. We describe our experience with severe invasive GAS infections in Ontario, Canada, during the past 5 years (February 1987 through December 1991). A case was defined as the isolation of GAS from blood or normally sterile tissue in association with hypotension (systolic blood pressure, < 90 mm Hg). Fifty cases were identified in patients ranging in age from 4 to 100 years (median age, 47 years); 29 (58%) of the patients died. A primary focus of infection was identified in 38 cases (76%), with soft tissue being the site involved most frequently (68%). No focus of infection was found in 12 patients, and 36 patients (72%) were bacteremic. Complications included acute respiratory distress syndrome (21 of 50), acute renal failure (20 of 50), hypocalcemia (19 of 24), elevated creatinine kinase values (21 of 27), coagulation abnormalities (15 of 21), and hepatitis (15 of 24). Eleven cases (22%) were nosocomial; one of these was secondary to another nosocomial case. Thirty-three isolates were available for M and T typing and for determination of the presence of the genes for streptococcal pyrogenic exotoxin (SPE). The most frequent types were M1T1 (10) and M12/T12 (8). Twelve isolates possessed the speA gene, and 16 isolates had the speC gene. Only three isolates possessed both speA and speC. All isolates possessed the speB gene.