Approximately 80% of human ovarian and endometrial cancers and 50% of breast cancers express GnRH and its receptor as part of an autocrine regulatory system. After binding of its ligand the tumor GnRH receptor couples to G-protein alphai and activates a variety of intracellular signaling mechanisms. (1) Through activation of a protein tyrosine phosphatase, autophosphorylation of growth factor receptors is reverted leading to an inhibition of mitogenic signaling and reduced cell proliferation. (2) Through activation of nuclear factor kappa B antiapoptotic mechanisms are induced protecting tumor cells from apoptosis induced, for example, by doxorubicin. (3) Through activation of the Jun kinase pathway AP-1 is induced, leading to cell cycle arrest in the G0/G1 phase. It seems reasonable to speculate that this system enables the tumor cell to reduce proliferation and to activate repair mechanisms while being protected simultaneously from apoptosis. Interestingly, GnRH antagonists show the same activity in this system as agonists, indicating that the dichotomy GnRH agonist-GnRH antagonist defined in the pituitary gonadotrope is not valid for the tumor GnRH system. Recently, a second type of GnRH receptor, specific for GnRH-II, has been identified in ovarian and endometrial cancers, which transmits significantly stronger antiproliferative effects than the GnRH-I receptor. GnRH antagonists have agonistic effects on this type II receptor. In animal models of human cancers, GnRH antagonists had stronger antitumor effects than GnRH agonists. Therefore, we performed a phase II clinical trial with the GnRH antagonist, cetrorelix (10 mg/day), in patients with ovarian or mullerian carcinoma refractory to platinum chemotherapy. Of 17 evaluable patients treated with cetrorelix, 3 obtained a partial remission (18%) which lasted for 2 to 6 months. Furthermore, 6 patients experienced disease stabilization (35%) for up to 1 year. In this very refractory patient population (median number of prior chemotherapies = 3) these results are quite remarkable when compared with palliative chemotherapy. In addition, cytotoxic GnRH analogs have been developed, where for example doxorubicin was covalently coupled to GnRH analogs. These compounds have superior antitumor effects in cancers expressing GnRH receptors as compared with native doxorubicin and allow for a targeted cytotoxic chemotherapy of gynecologic and breast cancers.