OBJECTIVE: We aimed to evaluate the current status of sexual maturation of Chinese children, to examine the association between obesity and early sexual maturation in boys and compare it with girls and to test the hypothesis that the associations differ by gender. STUDY DESIGN: Cross-sectional study. SUBJECTS: A representative sample involving 9812 boys and 8895 girls aged 6-18 years who participated in the Chinese Children and Adolescent Metabolic Syndrome Epidemiologic Study (July 2009- July 2010) were surveyed. METHODS: All subjects had complete anthropometry and sexual maturation data. Subjects who reached Tanner stage 2 or more (5601 boys and 6538 girls) were divided into tertiles based on the timing of sexual maturation. The subjects in the earliest tertile were included into the early-maturing group, and the middle tertile and the latest tertile into the not early-maturing group. Overweight was defined as a body mass index (BMI) >= 85th percentile and obesity >= 95th percentile. Logistic regression analysis was used to test how early maturation affected the risk of overweight. Multiple linear regression was used to examine the association between fatness (BMI Z-score) and sexual maturation. RESULTS: Slightly more boys were obese than girls (P < 0.01). The median age for girls of the Tanner stage 2 was 9.69 years, and for boys of Tanner stage 2 was 11.25 years. BMI Z-score were higher (P < 0.01) in both early-maturing girls and boys, compared with the non-early maturers,respectively. Early sexual maturation was positively associated with obesity in both girls and boys. With covariates adjusted and using non-early maturing as the reference group, odds ratios for combined overweight were 1.48 for boys and 2.64 for girls, and for obesities were 1.61 for boys and 3.49 for girls, respectively. CONCLUSION: Obesity is positively associated with sexual maturation in both boys and girls, and the association does not differ by gender, but the association is stronger in girls than in boys.