 In order to investigate whether our cell line data on tibolone action could be confirmed in vivo, and because relatively short-term effects of tibolone and other hormonal treatments in patients have never been described, postmenopausal women were treated for 21 days with tibolone, estrogen-only, or estrogen together with progestagen estrogenprogestagen. From these women sera were obtained and endometria were removed in order to assess the histological and immunohistochemical, biochemical hormone levels and molecular gene expression effects of relatively short-term tibolone use 21-days. The overall results may help us to obtain insight in the mechanism of action of tibolone on the human endometrium compared to reference preparations. 