PURPOSE: Leptin is a highly hydrophilic 16-kDa protein which is produced in the adipose tissue and participates in the regulation of food intake and energy expenditure. The aim of the present study was to examine the relation between umbilical cord blood leptin concentration and intrauterine growth. METHODS: Ninety-seven full-term newborn infants who were born in Yeungnam University Hospital from July to August 1998 were included in the study. They were divided into 3 groups related to birth weight : appropriate for gestational age(AGA) group(n=73), large for gestational age(LGA) group(n=17), small for gestational age(SGA) group(n=7). Birth weight, head circumference, mid-arm circumference, mid-arm circumference to head circumference ratio, Ponderal index, and BMI were measured at birth. Maternal body weight and placental weight were measured. Leptin concentrations of cord blood and maternal serum were measured by a RIA method, and testosterone, estradiol, insulin, c-peptide, glucose, white blood cell, hemoglobin, platelet count of cord blood were also measured. RESULTS: Leptin concentration in cord blood was positively correlated to birth weight and body length. Leptin concentrations(microgram/L) in cord blood were significantly different among groups(10.1+/-1.1 in LGA group, 8.7+/-0.9 in AGA group, 1.7+/-0.1 in SGA group). There was a statistically significant difference in leptin concentration of cord blood between female and male infants(11.6+/-1.9, versus 6.7+/-0.9). There was no significant correlations between leptin concentration of cord blood and placental weights or maternal leptin concentration. Therefore leptin concentration of cord blood can not inflect maternal leptin concentration but intrauterine fetal growth. CONCLUSION: Leptin in cord blood might originate mainly from fetal adipose tissue rather than the placenta, and may be related to fetal growth.