Androgen receptors are found in the endometrium and in the mammary gland as well as in a high percentage of malignant tumors derived from these tissues. Breast cystic disease, which is a frequent disorder in premenopausal women, predisposes to the development of breast cancer. In this disease androgens stimulate the production of secretory proteins like GCDFP-15, which accumulate in the cystic lesions. Elevated testosterone levels are correlated with an increased risk of breast cancer in pre- and postmenopausal women, but it is not known by which mechanisms androgens can favour breast cancer development. On the other hand androgenic compounds have been successfully used in the treatment of patients with metastatic breast cancer. The treatment with androgens points to an inhibitory role of these steroids in advanced breast cancer. In vitro, direct effects of androgens on mammary cancer cells have been studied in detail. The proliferation of ZR-75-1 and MFM-223 cancer cells is inhibited by androgens, while EFM-19 and MDA-MB-453 cells are stimulated under comparable culture conditions. MFE- 296 endometrial cancer cells are inhibited by androgens. The inhibitory effect is mediated directly via the androgen receptor. In the case of stimulatory activity the direct action via the androgen receptor can be involved. Indirect actions like binding to the estrogen receptor or metabolisation to estrogen-like compounds are also described. The divergent responses of mammary cancer cells depend on the receptor content but are not yet understood in detail.