Objective: to investigate the relationship between leptin concentrations, gonadotropic hormone concentrations, and body composition during puberty in a Dutch children cohort. Design: In a cohort of 98 children, we determined anthropometric measurements, body composition, and concentrations of leptin, FSH, and LH. Results: sex differences were observed from Tanner stage 1 onwards in weight, body fat percentage, and leptin/fat mass ratio. In boys and girls the relationship between leptin concentrations and fat mass was weaker at Tanner stage 2 (R 2 =0.33 and R 2 =0.39, p<0.001), 3 (R 2 =0.27 and R 2 =0.36, p<0.002), and 4 (R 2 =0.21 and R 2 =0.28, p<0.03) than at Tanner stage 1 (R 2 =0.51 and R 2 =0.67, p<0.001) and 5 (R 2 =0.46 and R 2 =0.78, p< 0.01). In girls, a peak in leptin concentrations (8.5+/-6.0 ng/ml) preceded a peak in LH and FSH concentrations (15.1+/-3.5 and 5.0+/-4.5 IU/l), and preceded a rise in estradiol and testosterone (7.2+/-30.8 and 0.88+/-0.3 ng/l). A lead/lag relationship was observed of leptin at Tanner stage 1 to LH and FSH at Tanner stage 2 (R 2 =0.12, p<0.05 and R 2 =0.18, p<0.05). In boys, there was no peak in leptin, LH, and FSH and additionally, leptin at Tanner stage 3 was related FSH at Tanner stage 4 (R 2 =0.17, p<0.04). Conclusion: in boys and girls during puberty factors independent of fat mass become (transiently) more important in the regulation of plasma leptin concentrations. Moreover, in girls leptin is suggested to act as a permissive factor for the onset of puberty, while in boys leptin has a different timing and possibly different function.+/-