PURPOSE: Measurement of antistreptolysin O (ASO) is often necessary to confirm a clinical diagnosis of recent streptococcal infection, especially in patients suspected of rheumatic fever and acute glomerulonephritis. Standard normal ranges for ASO should be established locally for each age group. We analyzed ASO to determine the upper limit of normal (ULN) ASO in children in the Gyeonggi-Incheon area. METHODS: ASO in normal individual concentrations were measured quantitatively by nephelometry on sera from 753 children (Male:381, Female:372). ULN were determined by separating the upper 20% from the lower 80% of the group (80 percentile). RESULTS: The mean ASO concentration calculated in a total cases was 149.9+/-7.2 IU/mL. The ASO concentration in neonates was 83.4+/-10.7 IU/mL, and lowest in the 1 year of age group, 26.7+/-6.6 IU/ mL, and increased to 318.0+/-33.2 IU/mL gradually in the 9 years of age group. Thereafter, ASO concentration decreased. The ULN for neonates was 122 IU/mL, for 0-3 years, 40 IU/mL; for 4-6 years, 113 IU/ mL; for 7-9 years, 489 IU/mL; for 10-19 years, 433 IU/mL; for 20-29 years, 122 IU/mL. CONCLUSION: The age-specific ULN for children in the Gyeonggi-Incheon area were determined. The distribution of ASO concentration according to age groups was different from previous reports. These results should be of clinical value to physicians to interprete the ASO results of their patients.