Our case concerns a 52-year-old male with FAP, who was treated surgically by restorative colectomy and ileal pouch anal anastomosis. Three years later, he presented with acute epigastric pain and obstructive ileum. While a mass in the left lateral abdominal region was palpated. The patient underwent laparotomy, some adhesions were dissected and biopsies were taken from the mass. Pathological examination revealed a desmoid tumor of the mesentery.