Tamoxifen is commonly used in the management of patients with breast cancer because of its anti-oestrogenic effects to the breasts. However, tamoxifen acts as an oestrogen agonist on the endometrium increasing the incidence of endometrial polyps, hyper­plasia and cancer. In addition, it may be possible that tamoxifen increases the occurrence of uterine body tumours. We describe a rare case of a large endometrial polyp with sarcomatous stromal components in a 73-year-old breast cancer patient treated daily with 20 mg of tamoxifen for four years. The glands of the polyp were lined by benign appearing epithelium. The polyp was twisted and protruded from a dilated cervix at the entrance of the vagina. The patient was treated by removal of the polyp and dilatation and curretage. A postoperative computed tomography scan showed many focal hypodense lesions in the hepatic lobes with a well-defined profile suggestive of metastatic disease and the patient was referred for combined chemotherapy. In conclusion, a case of a mes­enchymal malignant neoplasm arising in the uterus of a breast cancer patient treated with tamoxifen is reported and its clinical, his­tological and immunohistochemical features are discussed. Also, the international literature is reviewed.