A case of müllerian adenosarcoma with sarcomatous overgrowth in a postmenopausal 66-year-old female patient after adjuvant tamoxifen treatment for breast carcinoma is described. The patient was asymptomatic and the neoplasm was detected by pelvic sonography. The diagnosis was based on the histological findings after curettage and complementary total hysterectomy with bilateral salpingo-oophorectomy. The association of tamoxifen use and development of mesenchymal neoplasms is discussed.