Objective: To demonstrate sexual dimorphism in serum leptin levels not only during puberty, but also in childhood in Japan. Design: Cross-sectional study. Setting: Hamamatsu-Hokuen study in Japan. Subjects: Body weight and height were measured in normal-weight Japanese children and adolescents (143 boys, 178 girls), and 161 boys and 129 girls whose percentage of overweight for the standard (%Wt) was more than+25%. Serum leptin levels were compared with %Wt. Subjects were divided into group 1 (6-10 y of age) and group 2 (11-15 y of age) according to their age. Results: In overweight subjects, leptin was more highly correlated with %Wt in boys of group 2 (r=0.67, P<0.0001) than group 1 (r=0.32, P=0.004). In girls as well, a correlation coefficient was greater in group 2 (r=0.67, P<0.0001) than group 1 (r=0.44, P=0.0011). In normal-weight boys, there was no significant correlation between serum leptin and %Wt both in groups 1 and 2. On the contrary, statistical significance was demonstrated in the correlation between serum leptin and %Wt in group 1 (r=0.31, P=0.0019) and group 2 (r=0.35, P=0.0014) as well as in the total normal-weight girls (r=0.28, P=0.0011). Conclusions: It is suggested that sexual dimorphism is present in serum leptin levels, especially when compared to weight, as early as during childhood. European Journal of Clinical Nutrition (2001) 55, 989-993 | 
