Background：Group A streptococci (GAS) cause various infections in the school children. The change of isolation rate of GAS between time interval was observed by repeated throat cultures and acquisition rate of new strain was investigated by comparing the serotypes of GAS.

Methods：Throat cultures were taken from the school children in Chungnam and Seoul. Second throat cultures were taken from 119 children in Chungnam after 1 month and from 59 children in Seoul after 4 months, who showed GAS in the first throat culture. Serotypings such as T, M and opacity factor typing were performed and compared against 40 children in Chungnam and 26 children in Seoul who grew GAS in both throat cultures.

Results：GAS were isolated from 57.1% (68/119) in Chungnam and 45.8% (27/59) in Seoul in the second throat culture. Different serotypes between first and second throat culture were 5 of 40 (12.5%) in Chungnam and 4 of 26 (15.4%) in Seoul, respectively.

Conclusions：Almost half of children contained GAS continuously until 4 months and acquisition rate of new serotypes was 14.0% during this time. When GAS is repeatedly isolated, serotyping was very useful to recognize whether the strain is same or not. (Korean J Clin Microbiol 1999;2:14~18)