The aim of the study was to determinate the serum leptin level, glucose concentration, lipids (total cholesterol, HDL-cholesterol, LDL-cholesterol, triglyceride) and thyroid hormones level (triiodothyronine, thyroid stimulating hormone and free thyroxine) in a group of prepubertal children with simple obesity. Fourfold higher leptin concentration in obese (23.3 +/- 11.8 ng/mL) in comparison to a group of slim children (6.8 +/- 2.7 ng/mL), (p<0.0001) was shown. There was no significant difference in serum glucose level (87.3 +/- 9.7 mg/dL) between both studied groups. In obese subjects we showed higher triglyceride, insignificantly lower HDL-C concentrations and invariable other lipid fractions. Mean values of triiodothyronine (T3) were 3.0 +/- 0.7 nmol/L in obese children and 2.8 +/- 0.8 nmol/L in slim children. On the contrary, mean values of thyroid stimulating hormone (TSH) were 2.5 +/- 1.7 mIU/L and 2.8 +/- 0.7 mIU/L in the group of obese and normal children respectively. Free thyroxine (FT4) concentrations in our obese children were lower (11.5 +/- 2.9 pmol/L) than in slim subjects (14.6 +/- 3.1 pmol/L) but were within the reference range. The results obtained indicate that in children with simple obesity there was no dyslipidemia connected with leptin and thyroid hormones levels. However, higher triglyceride and lower HDL-C concentrations suggest a necessity of monitoring the body mass index and lipid profile in these patients.