Streptococcus pyogenes causes a variety of diseases ranging from mild pharyngitis to severe toxic shock syndrome (TSS) and acute rheumatic fever. Since 1987 there has been a resurgence of severe group A streptococcus infections including TSS, necrotizing fasciitis, and myositis. Using molecular and serotyping procedures, we recently studied two clusters of group A streptococcus disease that occurred within separate family units. The first cluster involved two family members (one with TSS and one with necrotizing fasciitis) and three health care workers who attended one of the index patients. The second cluster included a mother (with necrotizing fasciitis of the hand) and her three children. Group A streptococci isolated from individuals within both cluster groups were serotype M3;T3/13/B3264, and pulsed field gel electrophoresis revealed that all isolates except one had identical fingerprints of Sma I-digested chromosomal DNA. The findings demonstrate the potential for spread of serious group A streptococcus disease among individuals and the need for barrier protection when health care workers are exposed to secretions from infected individuals.