Polycystic ovarian syndrome is a heterogeneous disorder characterized by abnormal function of the hypothalamic-pituitary-gonadal axis, excessive production of androgens, aberrant intermediary metabolism, and structural changes in ovarian morphology. Long-term administration of the GnRH agonistic analogues brings about pituitary gonadotroph down-regulation after an initial period of stimulation of FSH/LH release. The resulting decrease in LH output and ovarian androgen production exerts beneficial effects on the clinical and biochemical parameters of the PCO syndrome. Ovarian volume and stroma usually decrease, but the results of treatment are not permanent, since relapse of the syndrome is usually observed a few months after cessation of the agonist. There are no serious side effects, and a small decrease in bone mineral content is recovered after discontinuation of the treatment. Possible indications for this regime include patients with very large ovaries and resistant hyperandrogenemia. On the other hand, the use of GnRH agonists in the preparation of PCOS patients for ovulation induction is already established.