A 37-year-old man with complaints of lassitude and slight lumbal pain, who had been found to have a low abdominal median mass and was referred to our hospital by a practitioner, was admitted for further examination. IVP, examination of the gastrointestinal tract by the oral procedure and re-examination of the colon by a double contrast procedure revealed deviation of the right ureter, the urinary bladder and the alimentary tract. This mass was found to be a retroperitoneal tumor with central necrosis by low abdominal CT scanning and the low abdominal echography. The mass was removed easily, though it was slightly adherent to the anterior surface of the sacral bone. Pelvic lymphadenectomy was also done simultaneously. The removed mass was encapsulated by fibrous tissue, round in shape, 750 grams in weight (13 by 12 by 12 cm), evenly flat, elastic soft and contained 230 ml bloody exudate at the center. Histologically this mass contained areas where oval and spindle cells made palisading arrangement and areas where the tumor cells had no communication with each other and stroma was edematous. However, hyperchromatism of nuclei of tumor cells and high cellularity indicated this mass to be a malignant Schwannoma. Dissected lymph nodes had no metastatic involvement. Since the surgical margin was detected to be invaded by tumor cells, postoperative prophylactic irradiation of Linac (10 Me V-X, total doses 4,750 rads) was performed on the whole pelvis. This patient has been well and has had no signs of recurrence of tumor for 23 months after the operation. Ninety-four cases of benign retroperitoneal Schwannoma and thirty-six cases of malignant retroperitoneal Schwannoma reported in Japan are reviewed.