Myxofibrosarcoma(MFS)is one of the most common sarcomas in the extremities of elderly patients.We report a rare case of MFS originating in the retroperitoneum.A 65-year-old man presented to our hospital with an intraabdominal tumor, which was identified by abdominal CT during an investigation of an abdominal protuberance.Contrast -enhanced CT and MRI revealed a 18×12 cm mass in the left retroperitoneum.The patient underwent retroperitoneum tumor resection.Operative findings showed that there was an indistinct area between the left side of the tumor and the abdominal wall.There was no residual tumor macroscopically after the operation.The histopathological diagnosis was MFS arising from the retroperitoneum. Four months later, local recurrence and para-aortic lymph node metastasis were diagnosed.We performed tumor resection; however, the recurrent tumor had invaded the iliopsoas and iliac muscles.Additionally, there was peritoneal dissemination around the tumor.Five months after the initial surgery, a recurrence on the back was identified.He died 7 months after the first operation.According to previous reports, patients who undergo an operation with pathologically negative margins achieve better surgical outcomes.Therefore, it is important to perform expanded resection with enough surgical margin as possible.