An association between the administration of tamoxifen in patients with breast cancer and the development of endometrial cancer has been suggested in a number of clinical studies, none of which was designed prospectively to study the endometrial effects of tamoxifen. Because of this observation, some have equated tamoxifen with "estrogen," and inferences have been drawn about its possible carcinogenic effect. This presentation analyzes the effect of tamoxifen on the lower genital tract in the adult human female, compares it with the effect of estradiol, and compares and contrasts the effect of these substances on the end organs of laboratory animals, cell lines, transplants, and their differential effects in the human uterus, cervix, and vagina. Of note is the fact that in patients who have not had breast cancer, there is no report of the development of endometrial cancer in women taking tamoxifen. Careful monitoring of this population may define with better precision the mechanism of action of tamoxifen as well as the process of oncogenesis in the human endometrium.