Objectives: The object of this study was to determine the relationship between body mass index (BMI) and early menarche in adolescent girls in Seoul. Methods: A retrospective study was conducted with 144 middle school students in Seoul who provided informed consent. We measured their body composition, and used the questionnaire survey method for data collection from November to December 2008. Past elemental body composition data were collected from elementary school health records of first year of middle school. Results: The early menarcheal group was taller and heavier than the late menarcheal group (p<0.05 from 8-12 years old). The body fat percentage (%), BMI were higher in the early menarcheal girls than the late-menarcheal girls (p<0.05, age at 13). In the result of multiple logistic regression, the BMI at the age of 8 and 9 was associated with early menarche after adjusting for birth weight, breast feeding and age at menarche of the mother (BMI at the age of 8: p for trend=0.01, BMI at the age of 9: p for trend=0.04). An increase in BMI from 7 to 8 year was associated with early menarche after adjusting for birth weight, breast feeding, age at menarche of the mother (p for trend=0.048). Conclusions: The BMI at the age of 8 and 9 was associated with the early menarche of girls and increase in BMI from 7 to 8 year was associated with the early menarche of girls. These results suggest that BMI and increase in BMI before menarche cause early menarche. Although this study does not represent all Korean adolescent girls, it is one of the few studies that have investigated the temporal relationship between BMI and early menarche.