OBJECTIVE: To compare the relationship between fasting serum leptin levels and body mass index (BMI) in children from different ethnic groups. SUBJECTS: Children aged 6 - 10 y from rural Gambia (n = 471) and central Italy ( n = 839). MEASUREMENTS: Anthropometry ( z- score of BMI) and fasting serum leptin concentrations. RESULTS: The Italian children had significantly higher mean BMI z- scores than the Gambian children ( males: Italy 1.58, Gambia - 1.44, Pless than or equal to0.0001; females: Italy 1.33, Gambia - 1.42, Pless than or equal to0.0001) and significantly higher serum leptin concentrations ( males: Italy 8.86 ng ml(-1), Gambia 1.78 ng ml(-1), Pless than or equal to0.0001; females: Italy 11.31 ng ml(-1), Gambia 2.22 ng ml(-1), Pless than or equal to0.0001). A significantly different relationship was observed between z- score of BMI and serum leptin levels in the Gambian and the Italian children for both boys and girls. CONCLUSION: A different relationship exists between z- score of BMI and leptin levels in these two groups of children from very diverse ethnic backgrounds. Future studies using detailed measures of body composition and energy balance are needed to help understand this relationship.