The hormone leptin produced in the adipose tissue is involved in the regulation of body weight. This study investigates whether plasma leptin levels are related to an infant's birthweight, and whether the levels change with feeding. We measured plasma leptin levels from infants who were large for gestational age (n = 21), small for gestational age (n = 21), and appropriate for gestational age (n = 20). Two blood samples were collected before and after breastfeeding from each infant and plasma leptin concentrations were determined by radioimmunoassay. Leptin concentration was found to be increased in large for gestational age infants and to be decreased in small for gestational age infants compared with the level in appropriate for gestational age infants. There was a positive correlation between plasma leptin levels and both the infants' birthweights and the body mass indexes. Plasma leptin concentrations were found to be decreased during fasting and to be increased after feeding (p < 0.01). It is concluded that the plasma leptin levels correlate with the size of adipose tissue mass and are related to the nutritional status.