A 26-year-old man was referred to the hospital for an abdominal tumor, which was detected at another hospital when he visited there because of abdominal distention. The patient was admitted to the hospital for further examination on November 13 1991. A retroperitoneal tumor in the presacral region was revealed by CT, ultrasonography, MRI, and angiography. Low grade malignant leiomyosarcoma deemed likeliest by pathological findings obtained from biopsy specimens. The patient underwent surgical operation on April 23 1992. Laparotomy showed the first right sacral foramen destroyed by the tumor, which also involved the sacral nerves. The greater part of the tumor, excepting a small portion of it in the sacral foramen, was extirpated to conserve those nerves. Macroscopically, the extirpated tumor was 325g in weight and 8×6×10cm in size. It had a yellowish white appearance and was well encapsulated in the cut section. Its histologic composition was spindle cells which were arranged in short bundles or interlacing fascicles. The tumor cells showed slight nuclear atypsim but no mitosis. Immunohistochemically, the tumor cells showed a positive stain for S-100 protein. The final diagnosis was Antoni A type benign schwannoma. The postoperative course has been uneventful to date, two years after the operation, with no growth of the remaining tumor being detected.