BACKGROUND Tamoxifen--a non steroidal triphenylethyl compound--in addition to having antiestrogenic properties may provoke weak estrogenic effects, the well known "paradoxical effects" on the female genital tractus. Concern has been raised about prolonged tamoxifen treatment and subsequent occurrence of endometrial adenocarcinoma; subsequent attention has been drawn through high risk histologic subtypes including poorly differentiated patterns and uterine sarcomas. METHODS We report two cases of uterine sarcoma arising in postmenopausal women taking tamoxifen, 20 mg daily during 38 and 42 months, for breast carcinoma: one leiomyosarcoma and one endometrial stromal sarcoma; both cases were asymptomatic and detected by pelvic sonography. CONCLUSIONS Further studies will be required to establish if there is a relationship between long term tamoxifen exposure and highly aggressive types of cancer of the uterine corpus exhibiting adverse histologic features such as uterine sarcomas. There is no consensus regarding uterine surveillance of women receiving tamoxifen. We advocate an annual gynecologic examination plus imaging by means of transvaginal ultrasonography.