Although AMH had long ago been identified as a hormone critical for normal development of the male reproductive tract, its importance in female reproductive function has only recently been established. A product of the granulosa cells (GCs) of the ovary at all stages of follicle development, it is most highly expressed in large preantral and small antral follicles of the human ovary. Studies in the mouse have highlighted the role of AMH as an inhibitory regulator of activation of primordial (resting) follicles in the ovary but it has also been shown to have an inhibitory function (suppression of FSH-stimulated estrogen production) in GCs of antral follicles. Circulating concentrations of AMH have proved to be a useful index of ovarian function both in identifying women with diminished ovarian follicle reserve and those with an abundance of follicles i.e. in women with polycystic ovary syndrome (PCOS). The results of recent studies, in our laboratory and in others, point to a role of AMH in the aetiology of PCOS which may involve action not only during early follicle development in the ovary but also, intriguingly, in the neural control of gonadotropin secretion.