In premenopausal women with ER and progesterone receptor (PgR) positive breast cancer, the preoperative endocrine therapy includes a combination of a gonadotropin-releasing hormone analogue (GnRH) plus tamoxifen. Recent studies suggest that neoadjuvant endocrine therapy with a combination of GnRH analogue and aromatase inhibitors (AIs: letrozole or anastrozole) is effective in selected premenopausal patients. The GnRH analogue, also known as a luteinizing hormone-releasing hormone agonist (LHRH agonist) or LHRH analogue, is a synthetic peptide drug modeled after the human hypothalamic gonadotropin-releasing hormone (GnRH). A GnRH analogue is designed to interact with the GnRH receptor and modify the release of pituitary gonadotropins, follicle-stimulating hormone (FSH) and luteinizing hormone (LH) for therapeutic purposes. Upon administration of a GnRH analogue, an initial stimulating action of the hypophysis occurs - termed a "flare" effect - which eventually causes a paradoxical and sustained drop in gonadotropin secretion. This second effect has been termed "downregulation" and can be observed after about 10 days. While this phase is reversible following cessation of medication, it can be maintained when GnRH agonists' use is continued for a long time. For a select group of patients, there is a delay of approximately 2-4 months before downregulation of the gonadotropins is observed.