Objective: To assess the effect of Tamoxifen (TAM) in the post menopausal endometrium, we retrospectively studied postoperative breast cancer patients, who were followed up at the Cancer Institute Hospital from Jan. 1, 1980 to Dec. 31, 1996.Study Design: Patients were divided in two groups: TAM-treated group (TAM group) and control group (non-TAM group). Eighty patients in the TAM group and 145 patients in the non-TAM group were compared cytologically and histologically. We used endometrial aspiration cytology.Results: 1. Histological findings (TAM group): The focal proliferation of endometrial glandular cells and stromal cells was found with atrophic endometrium, and stromal edema was also found. These endometrial changes were reversible, because these findings disappeared after treatment with TAM. These findings were different from the endometrial proliferative disorders.2. Cytological findings (TAM group): Endometrial glandular cells and stromal cells became larger, and were arranged in a sheet. Subnuclear vacuolization was seen, which looked like the early secretory phase endometrium. These changes were reversible, and different from the endometrial proliferative disorders.3. In the non-TAM group, we found atrophic endometrium.4. In the TAM group, we found 7 endometrial proliferative disorders, while in the non-TAM group, we found 5 endometrial proliferative disorders.Conclusion: TAM showed an estrogenic effect in the endometrium of post menopausal breast cancer patients.