OBJECTIVE: To study the serum leptin level and its relation to physical development in children, aged between 3 and 9 years, from an undeveloped region of China. METHODS: A total of 280 children aged between 3-9 years from a kindergarten and an elementary school of an undeveloped county of Hubei Province (147 boys and 133 girls) were enrolled in this study. They were divided into four groups according the age and gender: 3-6 year-old boy / girl group and 6-9 year-old boy / girl group. Their weights and heights were measured and body mass index (BMI) and body fat precentage(BFP) were calculated. The serum leptin level was detected using ELISA and was compared between the groups. The correlation between the serum leptin level and physical development parameters was evaluated. RESULTS: Serum leptin levels in the girls were significantly higher than those in the boys in the same age group (P <0.05), but no differences were found between children aged 3-9 years of the same gender. The leptin level was obviously correlated with BMI and BFP in both boys and girls aged 6 to 9 years (P<0.05), but not in children aged 3 to 6 years. CONCLUSIONS: The serum leptin level was different between boys and girls in children aged 3 to 9 years in the undeveloped region. It was correlated to BMI and BFP in children aged 6 to 9 years.