For about 20 years the antiestrogen tamoxifen is successfully used in the adjuvant and palliative treatment of estrogen receptor-positive breast cancer. Randomised trials showed an increased risk for the development of endometrial cancer related to tamoxifen. There is an 2-8fold increased relative risk. The estrogen-activity results in endometrial changes, like hyperplasia or carcinoma. On the contrary the benefit of tamoxifen in the adjuvant treatment overweighs its risks. Nevertheless the effects of the substance on the endometrium make patients feel uncomfortable. In this review we explain mechanisms of tamoxifens action on the endometrium. A review of literature shows the latest skills of clinical relevant endometrial changes and gives practical advice for the approach to patients using tamoxifen.