Between January 1975 and December 1983, 24 of 183 patients (13%) with familial adenomatous polyposis (FAP) seen at the Mayo Clinic had mesenteric fibromatosis (MF). MF was found most often in FAP patients with associated extra-colonic "Gardner" signs (19 patients) and those who had had previous abdominal surgery (20 patients). In 4 patients, MF appeared spontaneously. The male-to-female ratio was 0.4, with a median age of 31 years in women and 37 years in men. Ten of 24 patients (42%) had been asymptomatic prior to diagnosis at time of surgery for FAP. Complications of the disease included intestinal or urinary tract obstruction. Minimal surgical manipulation seemed to be associated with fewer postoperative complications and a lesser risk of regrowth of the tumor. Nonsurgical treatment, including tamoxifen and sulindac in combination, may be beneficial. Surgery should be reserved for relief of obstruction, and bypass is preferred to resection.