Success of assisted reproductive techniques, as determined by the response to hormonal treatments and embryo quality, can successfully be predicted by the concentration of anti-Müllerian hormone (AMH) in plasma of several species. Being able to predict ovarian follicular reserve of prepubertal female horses (fillies) would help to select fertile broodmares and reduce costs associated with animal upkeep. The objectives of this work were to (1) assess AMH dynamics in female horses during the first year of life and (2) determine whether AMH concentrations detected in plasma of prepubertal fillies are correlated with AMH concentrations and antral follicle count (AFC) after puberty. Warmblood fillies (n = 14) born from February to May of 1 year in the same stud were used. Blood samples for AMH determinations were collected from birth onward every 4 weeks up to the age of 1 year. At 2 years, blood samples were collected and AFC was determined by transrectal ultrasonography. The AMH concentrations were determined by ELISA (AL-115, Ansh Laboratories, Webster, TX, USA). Transrectal ultrasonography was used to determine the AFC, which corresponds to the total number of antral follicles detected with ultrasound. Statistical analysis was done with the SPSS Statistics 24 software (SPSS Inc., Chicago, IL, USA). The AMH was detectable in the plasma of all animals from birth onward. At birth, mean AMH concentration was 4.5 ± 1.2 ng/mL. The AMH concentration increased and peaked between 24 weeks (8.7 ± 4.4 ng/mL) and 28 weeks (6.7 ± 2.1 ng/mL) and subsequently decreased again (52 weeks: 3.9 ± 0.9 ng/mL). Very high variation among individuals during the first year was lost at 2 years of age. The AMH concentration at 2 years was highly correlated with AMH concentration at birth (r = 0.62, P < 0.05) and with AFC (r = 0.78, P < 0.001). Also, AMH concentration (r = 0.73, P < 0.01) and AFC (r = 0.6, P < 0.05) at 2 years were highly correlated with AMH concentrations at 24 and 28 weeks. Gestational length (337 ± 1 days), parity of the dam (4.6 ± 0.8), and placental weight (6983 ± 352 g) did not influence AMH concentrations at any time. Our results demonstrate that AMH is detectable in blood of female horses from birth onward. Despite its high variability between individuals up to 52 weeks, strong correlations were observed during the first 2 years of life. High correlations to AFC at 2 years suggest that determination of AMH in prepubertal female horses helps to predict the ovarian reserve and fertility in postpubertal life.