Context: In adult women, Anti-Müllerian hormone (AMH) is produced by small growing follicles, and circulating levels of AMH reflect the number of antral follicles as well as primordial follicles. Whether AMH reflects follicle numbers in healthy girls remains to be elucidated. Objective: To evaluate if serum levels of AMH reflects ovarian morphology in healthy girls. Design: Population-based cohort study. Setting: General community. Participants: 121 healthy girls aged 9.8 - 14.7 years. Main outcome measures: Clinical examination, including pubertal breast stage (Tanner´s classification B1 - 5). Ovarian volume as well as the number and size of antral follicles were assessed by two independent modalities: A) Magnetic resonance imaging (MRI): Ellipsoid volume, follicles ≥ 2mm, and B) Transabdominal ultrasound (TAUS): Ellipsoid- and 3D volume, follicles ≥ 1mm. Circulating levels of AMH, inhibin B, estradiol, FSH and LH were assessed by immunoassays; testosterone and androstenedione by LC-MS/MS. Results: AMH reflected the number of small (MRI 2 - 3mm) and medium (4 - 6mm) follicles (Pearson´s Rho (r) = 0.531 and r = 0.512, p<0.001) but not large follicles (≥ 7mm) (r = 0.109, p=0.323). In multiple regression analysis, small and medium follicles (MRI ≤ 6mm) remained the main contributors to circulating AMH (Beta 0.501, p<0.001) whereas the correlation between AMH and estradiol was negative (Beta -0.318, p=0.005). In early puberty (B1 - B3), the number of AMH-producing follicles (2 - 6mm) correlated positively with pubertal stages (r=0.453, p=0.001), whereas AMH levels were unaffected (-0.183, p=0.118). Conclusions: Similarly to adult women, small and medium antral follicles (≤ 6mm) were the main contributors to circulating levels of AMH in girls.