Leptin is secreted from the edipose tissue and has an important role in the regulation of energy metabolism. This study aimed to compare serum leptin levels of preterm and full-term infants during the first three months of their life and to define the roles of sex, weight, thickness of subcutaneous adipose tissue, gestational age and maternal leptin in the determination of serum leptin levels. Forty-four full-term and 32 preterm infants were included in the study. Weight, thickness of subcutaneous adipose tissue, serum glucose, cortisol, insulin and leptin levels were compared between preterm and full-term infants at 7th, 30th and 90th days. ELISA method was used in determining serum leptin levels. Weight, thickness of subcutaneous adipose tissue and serum leptin levels were significantly increased in full-term infants compared to preterm infants at days 7 and 30. At 90th day weight and thickness of subcutaneous adipose tissue were significantly increased in full-term infants, but the difference in serum leptin levels did not reach statistical significance (p=0.56). Weight was the most important factor predicting serum leptin levels at the 7th day. On the other hand, the thickness of subcutaneous adipose tissue was the most important determinant at days 30 and 90. Maternal serum leptin level was a determinant of serum leptin level at day 7. Sex was a determinant of serum leptin level of the infant at days 7 and 30. The differences in weight gain, increase in thickness of subcutaneous adipose tissue and increase of serum leptin levels were not significant between groups. But the increase in serum leptin levels was correlated in both preterm and full-term infants with weight gain and increase in thickness of subcutaneous adipose tissue. At three months of age, in the catch-up growth period, preterm infants reach serum leptin levels near those of full-term infants. The thickness of subcutaneous adipose tissue has a role in the determination of serum leptin levels after 30 days of life.