The cyclin-dependent kinase inhibitor, p27, has been shown to mediate cell growth arrest thereby significantly reducing the percentage of proliferating cells. It seems that p27 expression is essential for the control of normal endometrial proliferation, and reduced or absent p27 expression may be an important step in endometrial carcinogenesis. Our aim was to demonstrate the effects of tamoxifen therapy on the expression of p27 protein in the endometrium of postmenopausal breast cancer patients. Fifty-three pre- and post-tamoxifen treatment endometrium samples were examined immunohistochemically using p27 antibody. Tamoxifen therapy (20 mg/day) for 60 days increased the expression of p27 protein in the endometrium of postmenopausal breast cancer patients. We conclude that tamoxifen therapy does not seem to be directly involved in the carcinogenesis of endometrial carcinoma since the expression of p27 is not decreased.