BACKGROUND: Pharyngeal group A streptococcal (GAS) emm type surveillance enhances understanding of the epidemiology of pharyngitis and invasive GAS disease and formulation of multivalent type-specific vaccines. In addition, such surveillance provides pre-GAS vaccine baseline data. We assessed geographic and temporal trends in GAS emm-type distribution among pediatric pharyngeal isolates collected systematically in the United States and Canada from 2000 to 2007. METHODS: We collected approximately 100 acute GAS pharyngitis isolates from each of 13 widely scattered sites (10 in the United States and 3 in Canada) annually for 7 seasons (2000-2007) from 3- to 18-year-old children. We assessed emm type and subtype by DNA sequencing and analyzed temporal and geographic trends. RESULTS: A total of 7040 US and 1434 Canadian GAS isolates were studied. The 6 most prevalent emm types (in descending order) were 1, 12, 28, 4, 3, and 2 in the United States and 12, 1, 28, 4, 3, 2, and 77 in Canada, constituting 70%-71% of isolates in each country; 10 emm types constituted 87%-89% total. Fifty-six emm types were identified in the United States, including 8 new types, and 33 types in Canada. Although a few types predominated nationally, marked variability among individual sites and at individual sites from year to year was observed. US-Canadian differences in type distribution were apparent. Twenty percent of isolates represented emm subtypes that differed slightly from reference types; 110 new subtypes were identified. An experimental 26-valent M protein vaccine covers 85% of pharyngitis isolates. CONCLUSIONS: Although overall US and Canadian emm type distribution was consistent and relatively few types dominated nationally, striking intersite and temporal variations within individual sites in prevalent emm types of GAS occurred. These results have important implications for the development and formulation of type-specific GAS vaccines.