The serum leptin concentration reflects the amount of adipose tissue in the body. Although fat deposition in the fetus in the third trimester markedly increases, the role of leptin during pregnancy has not been clarified. In the present study, whether or not the serum leptin concentration correlates with growth in utero was investigated, in addition to how leptin levels change in the first few days after birth. One hundred sixteen Japanese infants were divided into term (n = 91) and preterm groups (n = 25). Term infants were divided into 3 subgroups: birth weight appropriate for gestational age (AGA) (n = 44), birth weight large for gestational age (LGA) (n = 28), and birth weight small for gestational age (SGA) (n = 19). Longitudinal changes in the concentration of serum leptin after birth were examined in 48 infants. The serum leptin concentration was determined by RIA. No significant difference in leptin levels between cord sera and infants' sera obtained within the first 6 h of life (n = 28) was observed. Within the first 6 h of life, the concentration of serum leptin in LGA infants (12.8 +/- 10.2 ng/mL) and SGA infants (1.6 +/- 1.1 ng/mL) was significantly higher and lower, respectively, than that in the AGA infants (4.4 +/- 3.0 ng/mL) (P < 0.01). A significant positive correlation was found between the leptin concentration within 6 h of life and birth body weight (r = 0.59, P < 0.01). After birth, the concentration of leptin in LGA and AGA infants significantly decreased to the level in SGA infants within 72 h [corrected] of delivery (P < 0.05). After 72 h [corrected] of life, no significant differences in the concentration of leptin were observed among the three groups, and low levels continued to 7 days of age. These findings indicate that serum level of leptin correlates with fetal body weight gain.