Magnetic resonance imaging of a 39-year-old woman who presented with an abdominal mass revealed a tumor with hemorrhagic lesions extending from the intrauterine space to the subserosa. Hysterectomy was performed for probable uterine sarcoma. The histological examination diagnosed uterine leiomyoma with severe myxoid degeneration and without malignant components. Hemorrhagic lesions were diagnosed as adenomyotic cysts, resulting in findings similar to those of a uterine sarcoma.