The adipokine leptin has been detected in human breast milk, but its effect on postnatal growth and development remains largely unclear. We hypothesized that leptin could affect infant's body weight gain during early lactation in the first 6 mo of life. Therefore, we evaluated leptin levels in maternal serum and breast milk of 23 healthy, lactating mothers and their neonates in a prospective, longitudinal study. Leptin concentration was quantified by a commercially available human leptin RIA. Our results showed that leptin levels in breast milk were 22-fold lower than in maternal serum, but both parameters were positively correlated to each other (r = 0.431, p = 0.001) and to maternal BMI (serum: r = 0.512, p < 0.001; milk: r = 0.298, p < 0.001) over 6 mo of lactation. A negative association was found between breast milk leptin levels during the first week after delivery and the infant weight gain from the end of the first to the sixth month (r = -0.681, p = 0.007). This suggests that milk-borne leptin provides a link between maternal body composition and infant growth and development and plays a critical role in regulating appetite and food intake during early infancy.