Uterine leiomyomas are the most frequent gynecological benign tumors. Their growth is regulated by ovarian steroids, therefore a hypoestrogenic state, like menopause or pharmacologically induced pseudo-menopause by GnRH-agonists or GnRH-antagonists, is associated with the decrease of their volume. This volume reduction allows a less invasive surgical procedure and may reduce the amount of blood loss during surgery. Therefore, GnRH-agonists and antagonists are used in presurgical treatment of uterine fibromatosis. This review analyses the effects of GnRH-agonists and GnRH-antagonists therapies. GnRH-agonists produce a down-regulation of GnRH receptor, while GnRH-antagonists compete with endogenous GnRH for pituitary binding sites. Due to the lack of any intrinsic activity of GnRH-antagonists, the characteristic initial flare-up observed with GnRH-agonist treatment is absent. So, GnRH-antagonists rapidly suppress gonadotropin release within 4-8 h, while GnRH-agonists show clinical effects after 2 or 3 weeks of treatment. GnRH-antagonist activity is dose-dependent so it is possible to adjust the dose to obtain the proper levels of inhibition. The GnRH-agonist presurgical treatment usually is a short-term therapy (3-6 months), because it causes side-effects like menopause symptoms. GnRH-antagonist clinical effects can be achieved with a short-time therapy too. Their side-effects include flushes and head-ache. After stopping therapy with both drugs, leiomyomas rapidly achieve their original size while side-effects disappear. Further studies are necessary to establish the use of GnRH-antagonists in leiomyomas therapy, but in Italy this is not possible because their use is not approved.