The majority of modern hormone replacement therapy (HRT) regimens contain estrogen and progestogens, given either in a cyclical or continuous manner. About 15% of the endometrial biopsies taken from women on sequential HRT show proliferative activity including atypical endometrial hyperplasia in up to 1% of the cases. The majority of biopsies from women under continuous combined HRT show an endometrial atrophy. About 2-3% of these women will present proliferative activity, usually without atypical hyperplasia. Contrary to breast cancer, an increased risk of endometrial cancer has not been reported in the WHI- and HERS-studies. However, endogenous factors, such as obesity, diabetes mellitus, the distribution of estrogen receptors alpha and beta and genetic polymorphisms for receptors and enzymes might alter the endometrial stimulation under different types of HRT. There should be a liberal indication for endometrial biopsies in Hereditary Non-Polyposis Colorectal Cancer (HNPCC). HNPCC-patients under HRT as well as for ultrasonographic evaluation of the endometrium. The risk of atypical hyperplasias or carcinoma under unopposed estrogen-therapy varies from 2 to 10%. So, this kind of HRT should not be used in non-hysterectomised women. As far as the risk of endometrial cancer under any kind of HRT is concerned, the different molecular pathways of endometrial carcinogenesis (type 1 and 2 cancers) should be taken into account. The use of tibolone leaves the endometrium unaffected.