In order to define serum leptin values in children, its concentration was assayed in 3,453 children, 5-14 years old, and body mass index (BMI) and pubertal development were recorded. Insulin, testosterone (in males) and 17beta-estradiol (in females), the sum of four skinfold thicknesses (SST), waist-to-hip ratio, and body fat mass were also determined in 1,601 children. Analysis of multicollinearity effects on estimated models demonstrated a quasi-linear correlation between SST and BMI, the former being prevalent. Although other variables were strongly correlated with leptin, assuming only SST as predictor, R2 yielded a value of 0.711 in males and 0.607 in females. When the other variables were added, R2 increased by about 0.03 in both sexes. BMI and SST were the most important of all the predictors and each can act as a sort of proxy for the others. When the z-scores of BMI of all 3,453 children were subdivided into deciles, any correlation with leptin was no more significant inside each BMI z-score range. This study demonstrates that subcutaneous fat mass may be considered the prevalent determinant factor. The adopted statistical procedure furnished results useful for reference values based on BMI z-score as a simple and appropriate evaluation for serum leptin concentrations in children.