To determine the relationships between serum leptin levels in the umbilical vein (UV) and artery (UA) and the anthropometry of mothers and neonates. Blood was taken from 61 pregnant women who were admitted for delivery and from the umbilical vein and artery just before delivery of the placenta. Leptin level was measured by immunoradiometric assay. Comparisons between serum leptin concentrations in UVs and both maternal and neonatal anthropometry were made according to neonatal sex. Mean leptin UA and UV concentrations in female infants were significantly higher than those in male infants (both, P=. 002). Leptin concentrations in UVs of the total infants were related to maternal body weight and body mass index preconceptionally as well as at birth, to neonatal birth weight, to gestational age, to Kaup index and to body fat content of the infants. A sex difference was observed not only in serum leptin concentrations UA and UV but also in the degree of significance between the relationship of cord leptin and both maternal and neonatal anthropometry. Also, the UA leptin level had a closer relationship to neonatal anthropometry, but the UV leptin level was more closely related to maternal anthropometry.