We report four cases of mixed müllerian tumors of the endometrium arising in postmenopausal women taking tamoxifen for breast carcinoma. Various histopathologic features were encountered: one müllerian adenosarcoma and three malignant mixed müllerian tumors so called carcinosarcomas (in one case the sarcomatous component was homologous composed of tissues normally found in the uterus, while the others were heterologous because they were containing some elements normally not found in the uterus). Concern has been raised about prolonged tamoxifen treatment and subsequent occurrence of endometrial adenocarcinoma. Then, attention has been drawn through high-risk histologic subtypes including poorly differentiated patterns, uterine sarcomas and such mixed müllerian tumors. There is no consensus regarding internal surveillance of women receiving tamoxifen. The usefulness of pelvic sonography has not been demonstrated.