Anti-Müllerian hormone (AMH) is known to be secreted in women by granulosa cells of growing follicles from the primary to the large antral follicle stage. Serum and intrafollicular AMH levels are elevated in patients with polycystic ovary syndrome (PCOS) due to increased number of small follicles and an increased secretion within each of these small follicles. This excess of AMH is strongly suspected to play a role in the follicular arrest of PCOS by a negative action on aromatase expression and on action of FSH. The interest of serum AMH assay is also clinically important because AMH level is more sensitive, more specific and reproducible from one center to another as the follicle count. Rates above 5ng/ml or 35pmol/l may be considered as a diagnostic criterion for PCOS.