Patients with central precocious puberty (CPP) may have increased serum leptin levels; however, it is not well known whether this increase differs between patients with and without obesity. Our objectives were to describe the changes in serum leptin in girls with CPP in the first 12 months after diagnosis based on body mass index (BMI) and to explore whether serum leptin level at CPP diagnosis is related to BMI z-score (BMIz) after a 1-year follow-up. A prospective cohort study was performed. We included 42 girls with idiopathic CPP in Tanner stages II and III. Anthropometric measurements were performed, and serum leptin was measured at study initiation and after 12 months. Patients were stratified according to BMI category (30 with a BMI in the 95th percentile). Study variables were compared. Correlations among leptin, BMIz, and body fat were assessed. Leptin increased gradually during the first year of treatment. In girls with a BMI in the