The occurrence of a primary leiomyosarcoma of the mesentery is rare. A 61-year-old man was admitted to the hospital complaining of an abdominal mass. The findings of both abdominal ultrasonography and a computed tomography (CT) scan revealed an irregular and heterogeneous mass located in the mesentery. A laparotomy was performed and a 7.0 x 6.5 cm tumor was thus found within the jejunal mesentery. The tumor was successfully resected by a combined resection of 40 cm of the jejunum and end-to-end anastomosis of the jejunum. Pathological examination of the resected specimen revealed leiomyosarcoma. The patient had an uneventful postoperative course, but multiple liver metastases were discovered 1 year and 5 months after the initial operation. A second operation was performed, but the patient died due to hepatic failure and unexpected bleeding from the cut surface of the remnant liver. Preoperative imaging examinations, including abdominal ultrasonography and CT scan, were thus found to be useful tools for both identifying and diagnosing the origin and extension of a mesenteric mass. However, even using such diagnostic techniques an accurate diagnosis of intraabdominal leiomyosarcoma remains difficult.