A 41-year-old man was admitted to the hospital complaining of mild epigastralgia. Radiographic examinations revealed a right intrapelvic solid tumor (5 x 3 cm) apparently causing stenosis of the right ureter at the level of pelvic brim. The patient underwent an operation. The tumor which invaded the ileum and ureter was excised with resection of the affected segment of ileum and nephroureterectomy. Histological diagnosis was mesenteric fibromatosis. Barium enema study revealed no polyposis of the colon postoperatively. One year after surgery the patient has no signs of recurrence.