Animal and 'in vitro' experiences learned that human chorionic gonadotropin (hCG) is capable to protect from breast cancer. Receptors for hCG/luteinizing hormone (LH) are present on human female and male breast cancer cells. hCG decreases proliferation and invasion of breast cancer MCF-7 cells by inhibiting NF-kappa B, AP-1 activation and other genes. Doxorubicin toxicity is enhanced by conjugation with beta-hCG in MCF-7 cells. All these pieces of evidence suggest that hCG is active in human breast cancer. Direct proof however is missing. We performed a pilot study phase I trial for testing the inhibitory effects or recombinant hCG (rhCG) on primary breast cancer. Twenty-five postmenopausal women with newly diagnosed breast cancers of more than 1.5 cm were biopsied before randomization to receive either 500 microg rhCG (n=20) or placebo. After 2 weeks, surgery was done and tissues were analysed with regard to morphological, immunohistochemical and biochemical changes in tissues and plasma. rhCG reduces significantly the proliferative index and the expression of both the oestrogen receptor and progesterone receptor. rhCG does not modify the hormonal level of estradiol, progesterone, inhibin and follicle stimulating hormone (FSH) but increases significantly the level of LH. In a second pilot study, we tested the clinical efficacy through an open-label single centre study in 13 postmenopausal women with metastatic breast cancer. A 500 microg rhCG once every 2 days shows activity in postmenopausal metastatic breast cancer. The time to progression is relatively short. Response to previous hormonal treatment is indicative for rhCG activity. Given the data in primary and metastatic breast cancer rhCG further large scale investigation is highly warranted. rhCG can be an realistic option in (chemo-) prevention trials.