We studied serum leptin levels in 189 healthy children to evaluate related factors during childhood and adolescence. Leptin correlated with body mass index (BMI), triceps skinfold thickness (p<0.001) and body weight (p<0.01). Obese children and girls had higher leptin levels than non-obese children and boys, respectively (p<0.001). In girls, leptin correlated positively with age, skinfold thickness and BMI (p<0.001). In boys, leptin correlated negatively with age (p<0.001) and positively with skinfold thickness (p<0.05). Prepubertal boys had higher leptin levels than prepubertal girls and pubertal boys (p<0.05). Pubertal girls had higher leptin levels than prepubertal girls and pubertal boys (p<0.001). Leptin levels in girls were higher at Tanner stages 4 and 5 than at stage 1 (p<0.001). In conclusion, serum leptin levels are related with adiposity, have obviously age-related gender differences during childhood and adolescence, and may be involved in the maturation of reproductive capacity.