Objective: To examine the influence of childhood obesity on the early onset of puberty and sex hormones in girls. Methods: Healthy girls with different percentages of body fat at baseline (40 obese, 40 normal, and 40 lean) were recruited from three elementary schools in Shenyang, China. These girls (mean age 8.5 years) were also matched by height, school grade, Tanner stage, and family economic status at baseline. Anthropometry, puberty characteristics, and sex hormone concentrations were measured at baseline and at each follow-up visit. The generalized estimating equation model and analysis of variance for repeated measures using a generalized linear model were used to determine the differences in puberty characteristics and sex hormones among three groups. Results: Over 4 years, mean age of breast II onset was earlier among obese girls (8.8 years) than normal girls (9.2 years) and lean girls (9.3 years). The prevalence (%) of early-maturation in the obese, normal, and lean groups was 25.9%, 11.1%, and 7.4%, respectively. Obesity was associated with an increased risk for breast stage II (year 2: RR, 6.3; 95%CI, 1.9-21.1 and year 3: RR, 6.9; 95%CI, 0.8-60.1). None of the girls experienced menarche in the first year; however, by the fourth year 50.0%of obese girls had menarche onset, which was higher than normal weight (27.5%) and lean girls (8.1%). Themean estradiol level increased with age in the obese, normal, and lean groups. The mean estradiol concentration was higher in obese girls than in normal and lean girls throughout the 4-year period (P<0.05). Conclusions: Childhood obesity contributes to early onset of puberty and elevated levels of estradiol in girls.