Summary: In this paper, we present a case of myxoid leiomyosarcoma development in a patient receiving tamoxifen for 3 years because of breast cancer. The myxoid leiomyosarcoma should be included in the differential diagnosis of any uterine tumor with a predominantly myxomatous composition. A review of the literature indicates that tamoxifen may increase not only the risk for endometrial cancer but also for uterine sarcoma, suggesting vigilance for uterine cancer in women who are being treated with this drug.