Background and Aims: Leptin, the product of the obese gene (ob), is synthesized by adipose tissue and contributes to the regulation of energy homeostasis and food intake. Recently, immunoreactive leptin was reported to be present in human milk. The objective was to determine if there was a relation between breast milk leptin concentrations and adiposity in exclusively breast-fed infants. Methods: Fifty healthy, exclusively breast-fed infants beyond neonatal period, and their mothers were included into the study. Infants whose weight-for-length was above the 90th percentile were defined as obese (n=17), and non-obese if the weight for length between 20-90th percentile (n=33). Anthropometric measurements of infants and mothers were also made and breast milk samples were analyzed for leptin. Results: There was no significant difference between breast milk leptin concentrations of obese and non-obese infants' mothers. Breast milk leptin concentrations were significantly correlated with mothers' body mass index when all subjects analyzed. There was no significant correlation between breast milk leptin concentrations and body mass index of infants. Conclusion: Leptin concentrations of human milk are not different in the mothers of obese and non-obese infants. These findings suggests that milk-borne leptin has no significant effect on adiposity during infancy. (C) 2002 Elsevier Science Ltd.