Few data are available regarding leptin levels in children with different pubertal stages or with precocious puberty (PP). The aim of this study was to assess the changes in serum leptin levels in patients with PP. We studied 20 girls with PP, with Tanner stage II-III; the age at the beginning of pubertal signs ranged from 4.2 to 7.1 yr; all the girls had an advantaged bone age. Controls were subdivided in two groups: group 1: 20 pre-pubertal girls with the same chronological age of the patients, without any signs of pubertal development (Tanner stage I); group 2: 20 additional girls with the same bone age, pubertal stage and body mass index (BMI) of the girls with PP. Serum leptin levels in females with PP are similar to those found in subjects with normal puberty (9.0 +/- 0.8 vs 9.1 +/- 0.9 ng/ml; ns) and different from subjects with the same chronological age without activation of puberty (5.6 +/- 0.9 ng/ml, p < 0.001). In all groups leptin levels correlated significantly with BMI (girls with PP: r = 0.5 1, p < 0.02; control group 1 girls: r = 0.71; p < 0.0001; control group 2 girls: r = 0.49; p < 0.02), there was no significant relationship between leptin and activation of hypothalamic-pituitary-gonadal axis. Our results indicate that the serum leptin levels in the girls with PP are not significantly different from levels in healthy girls at a similar stage of pubertal development, suggesting that the relationship between serum leptin levels and BMI is also present in this pathological situation.