In a retrospective study of invasive infections due to group A Streptococcus (GAS) in Stockholm during 1987 to 1995, the average incidence per 100,000 residents per year was 2.3, varying between 3.7 per 100,000 (in 1988) and 1.3 per 100,000 (in 1993). Incidence was 1.8 in the age group of 0-4 years but otherwise increased by age, from 0.48 in the age group of 5-14 years to 6.1 among those over 65 years of age. A review of 151 invasive episodes occurring in 1983-1995 showed cyclic increases of infections due to T1M1-serotype strains during 1986-1990 and 1993-1995. The T1M1 serotype accounted for 27 (20%) of 135 available GAS strains. Streptococcal toxic shock syndrome (STSS) developed in 19 (13%) of the 151 episodes. The case fatality rate was 11% overall but 47% among patients with STSS. In a multivariate logistic regression model, STSS was associated with a history of alcohol abuse (odds ratio [OR], 6.3; P = .004) and infection with a T1M1 strain (OR, 6.7; P = .007). Case fatality was associated with age (OR, 14.5; P = .08), immunosuppression (OR, 4.7; P = .02), and STSS (OR, 21.5; P < .0001) but not with T1M1 infection. Hypotension was significantly associated with a fatal outcome, regardless of whether STSS developed (P < .0001).