Background . The explanation for the very substantial decrease in the incidence of acute rheumatic fever in the United States, particularly over the past 50 years, is unclear. It has been proposed that certain M types of group A streptococci (GAS) include strains that are particularly rheumatogenic and that others are nonrheumatogenic.

Methods . We compared the M type distribution of GAS recovered from children from Chicago, Illinois, with acute pharyngitis during 1961–1968 to that of GAS recovered from Chicago children and children from across the United States in 2000–2004, with attention to changes in M types that previously were associated with rheumatogenic strains.

Results . The rheumatogenic types 3, 5, 6, 14, 18, 19, and 29 comprised 49.7% of 468 pharyngeal isolates during 1961–1968 but only 10.6% of 450 Chicago isolates during 2000–2004 (P < .001) and 17.9% of 3969 isolates nationwide during 2000–2004 (P < .001). Significant decreases in types 3, 5, and 6 and virtual disappearance of types 14, 18, 19, and 29 occurred between the 2 study periods. No change in the proportion of type 1 isolates, a highly heterogeneous group that includes some rheumatogenic strains, was observed. The nonrheumatogenic GAS types 2, 4, 22, and 28 increased from 4.9% to ∼28% of pharyngeal isolates in Chicago and nationwide between the 2 study periods (P < .001).

Conclusions . These data support the concept of rheumatogenic strains of GAS and indicate that the marked decrease in the incidence of acute rheumatic fever in the United States over the past 4 decades is correlated with the replacement of rheumatogenic types by nonrheumatogenic types in cases of acute streptococcal pharyngitis in children. The reasons underlying the observed change in distribution of M types remain to be elucidated.