Malignant transformation of a uterine leiomyoma is an exceedingly rare phenomenon. Previous reported examples of malignant neoplasms to arise so include leiomyosarcoma, angiosarcoma, and malignant mesenchymoma. We report an intramural carcinosarcoma arising in a uterine leiomyoma in a 38-year-old woman. This case is unique in being the first recorded carcinosarcoma arising within a uterine leiomyoma and the first example of an epithelial neoplasm to do so. We discuss the differential diagnosis and possible pathogenesis of the tumor that has likely arisen secondary to epitheliogenesis in a mesenchymal neoplasm.