The expression of the vitamin D receptor (VDR) and enzymes involved in the metabolism of vitamin D in female reproductive tissues, suggests that vitamin D is involved in the physiologic reproduction process. VDR is expressed in granulose cells (GCs) and cumulus oophorus cells of human ovarian tissue, in decidua and placenta, in endometrium and fallopian epithelial cells and in the pituitary gland. Further, the human cultured syncytiotrophoblasts express VDR, which stimulates the expression and the secretion of human chorionic gonadotropin (hCG). The human placenta, the endometrium and the ovary express also 1α- hydroxylase (encoded by CYP27B1), indicating that these tissues are able to synthesize locally the 1,25 dihydroxyvitamin D3 [1,25(OH)2D3, or calcitriol], which is the active form of vitamin D. Vitamin D deficiency is hypothesized to contribute to the pathophysiology of a spectrum of gynecological disorders, of which polycystic ovary syndrome (PCOS) appears to be most well studied. An overexpression of VDR and vitamin D metabolizing enzymes is also described in the peritoneal lesions and in endometrial tissue of women with endometriosis compared with healthy controls. Serum 25 hydroxyvitamin D levels are reported to predict ovarian response in women undergoing ovulation induction with clomiphene citrate (CC). Low levels of 2m 25 hydroxyvitamin D levels and vitamin D deficiency (25 nmol/l or 10 ng/ml) were found to be associated with lower rates of follicle development and pregnancy after ovarian stimulation with 50 mg CC; of note, the threshold taken to define vitamin D insufficiency in this latter study is consistent with a state of severe vitamin D deficiency.