To see whether changes in the epidemiology of group A streptococcal disease in the USA have been accompanied by a corresponding change in serotype distribution, epidemiological and M-typing and T-typing data for 5193 strains sent to the Centers for Disease Control, Atlanta, between 1972 and 1988 were analysed. The proportions of M-types 1, 3, and 18 increased significantly during the study period. These M-types were more likely to be invasive, to cause fatal infection, and to occur in a cluster of infections than were other types. By contrast, the proportions of M-types 4 and 12 decreased; they were less invasive and were less likely to be found in clusters than were other types. These data suggest that changes in the epidemiology of group A streptococcal disease may be related to changes in the distribution of M-types causing infection.