Inhibin B is predominantly produced by early healthy antral follicles during the luteo-follicular transition of the menstrual cycle. High inhibin B concentrations during the early follicular phase are responsible for the decline in FSH serum levels closing the FSH window and assuring single dominant follicle selection in the human. Early follicular phase inhibin B levels decrease over time, reflecting the recruitment of a diminished cohort of follicles with ovarian ageing. Hence, inhibin B is a predictor of poor response in IVF. In patients with PCOS inhibin B levels (potentially representing the number of healthy, early antral follicles) may be associated with the severity of ovarian dysfunction and consequently may predict ovulation induction outcome. However, inhibin B levels are normal in most PCOS patients suggesting a normal number of healthy follicles despite an increase in overall follicle number. Recent findings indicate that initial inhibin B concentrations can not predict the outcome of ovulation induction by either clomiphene citrate or FSH. Finally, inhibin B levels decrease over time in PCOS.