We investigated the changes in the serum leptin concentration from birth to 30 days of age in order to determine the effects of leptin in early infancy. Twenty-seven Japanese term infants (12 boys and 15 girls) were examined. The serum leptin concentration at 3–6 days of age was significantly lower than that in cord blood (p < 0.0001) and was significantly higher at 30 days than at 3–6 days (p < 0.0001). The serum leptin concentration did not differ with gender or nutrition. The body weight gain and the magnitude of leptin rise that occurred between 3–6 and 30 days of age showed a significant positive correlation (r = 0.79; p < 0.0001). These changes in leptin levels suggest that leptin plays an important role in the regulation of weight gain in early infancy.