PURPOSE: The aim of this study is to evaluate the relationship between initial body mass index (BMI) and BMI after gonadotropin-releasing hormone agonist (GnRHa) treatment in idiopathic true precocious puberty girls. METHODS: The subjects were 99 idiopathic true precocious puberty girls treated with GnRHa for more than 1 year. The patients were categorized into two groups according to initial BMI; normal weight group (BMI < 85 percentile) and overweight/obesity group (BMI > or = 85 percentile). We investigated chronologic age (CA), bone age (BA), BA advancement (BA-CA), height (Ht), Ht-standard deviation score (Ht_zs), BMI, BMI_zs, predicted adult height (PAH), and PAH_zs before initiation of GnRHa treatment and 1 year later. RESULTS: Baseline CA, BA, BA-CA, Ht_zs, and PAH showed no differences between normal weight group and overweight/obesity group. BMI_zs increased only in normal weight group, and DeltaBMI_zs was negatively correlated with initial BMI_zs (r = -0.501, P < 0.001). PAH_zs increased less in normal weight group (DeltaPAH_zs = 0.30) than in overweight/obesity group (DeltaPAH_zs = 0.66) (P = 0.02), but there was no correlation between initial BMI_zs and DeltaPAH_zs. DeltaBA-CA and DeltaHt_zs were not different between two groups either. Comparing patients with increased BMI_zs with those whose BMI_zs decreased or remained the same, there were no differences in DeltaBA-CA, DeltaHt_zs, and DeltaPAH_zs. On multiple regression analysis, DeltaBMI_zs was negatively correlated with initial BMI_zs, and it showed no correlation with CA, BA, BA-CA, height, and dose of GnRHa. CONCLUSION: BMI_zs increased after 1yr of GnRHa treatment in idiopathic true precocious puberty girls whose initial BMI_zs was normal, and its increment was negatively correlated with initial BMI_zs.