The aim of the study was to determine the serum leptin level and anthropometric indices (body mass index - BMI) in a group of prepubertal children with simple obesity. These parameters were compared with the values characteristic for the group of slim children. The study showed that serum leptin levels were significantly higher in obese children compared with the group of slim children (p<0.0001). There was a positive correlation (r=0.48; p<0.05) between leptin levels and nutritional status indices expressed by BMI. Results of this study suggest that leptin level in the group of children with simple obesity does not have an effect on satiety, on reduction of appetite or on the increment of adipose tissue. The study showed also that hyperleptinemia might be an effect of leptin resistance.