Background: Regulation of body composition during childhood is complex. Numerous hormones are potentially involved. Leptin has been proposed to restrain weight gain, but results are inconsistent. Objectives: We examined if baseline fasting levels of ghrelin, adiponectin, leptin, insulin, insulin-like growth factor I (IGF-1), osteocalcin and intact parathyroid hormone (iPTH) were associated with body composition cross-sectionally and longitudinally in 633 8-11-year-olds. Design: Data on hormones and body composition by Dual-energy X-ray absorptiometry from OPUS School Meal Study were used. We looked at baseline hormones as predictors of baseline fat mass index (FMI) or fat-free mass index (FFMI), and also subsequent changes (three and six months) in FMI or FFMI using models with hormones individually or combined. Results: Cross-sectionally, baseline leptin was positively associated with FMI in girls (0.211 kg/m(2) pr. μg/ml (0.186; 0.236), p<0.001) and boys (0.231 kg/m(2) pr. μg/ml (0.200; 0.261), p<0.001). IGF-1 in both genders and iPTH in boys were positively associated with FMI. An inverse association between adiponectin and FFMI in boys and a positive association between IGF-1 and FFMI in girls were found. In longitudinal models, baseline leptin was inversely associated with subsequent changes in FMI (-0.018 kg/m(2) pr. μg/ml (-0.034; -0.002), p=0.028) and FFMI (-0.014 kg/m(2) pr. μg/ml (-0.024; -0.003), p=0.006) in girls. Conclusions: Cross-sectional findings support that leptin is produced in proportion to body fat mass, but the longitudinal observations support that leptin inhibits gains in FMI and FFMI in girls, a finding which may reflect preserved leptin sensitivity in this predominantly normal weight population.