Objectives: 1. To evaluate the relationship between plasma leptin and TNFα concentrations in obese children and to assess the differences between hyperinsulinemic and normoinsulinemic groups. 2. To evaluate the relationship between plasma leptin and insulin levels in obese children. 3. To investigate the TNFα G308A mutation in obese children. Methods: Body mass index (BMI), fasting plasma glucose and insulin levels, oral glucose tolerance test results, homeostasis model assessment of insulin resistance (HOMA-IR) results, and plasma leptin and TNFα concentrations were evaluated in obese children (n = 45) and age- and gender-matched, lean healthy controls (n = 40). Results: In obese children the fasting insulin, HOMA-IR results, plasma leptin and TNFα concentrations were significantly higher than in controls (p <0.05). Furthermore, obese females showed higher plasma leptin and insulin resistance compared to obese males. While plasma leptin, TNFα levels and HOMA-IR results were similar in the prepubertal and pubertal groups, insulin levels were significantly higher in the pubertal group. Plasma leptin and TNFα concentrations were similar in hyperinsulinemic and normoinsulinemic obese children. In control children, plasma leptin concentrations showed a positive correlation with BMI, age, fasting insulin and HOMA-IR results. In obese children, plasma leptin levels did not correlate with BMI, fasting insulin or TNFα. Conclusion: Plasma leptin concentrations did not show any correlation with TNFα levels in obese children. Furthermore, plasma leptin and TNFα concentrations were similar in hyperinsulinemic and normoinsulinemic obese children. © Freund Publishing House Ltd., London.