We studied the clinical, gross, and histologic findings of 130 fibromatoses of the mesentery and other peritoneal sites. Seventeen patients had Gardner syndrome, 12 had prior abdominal surgery, and six had apparent estrogen elevation, including five pregnant or postpartum women and an alcoholic male with gynecomastia. The tumors were usually large and grossly circumscribed. Most often, they were located in the mesentery of the small bowel. They were multiple in 18 cases. Typical histologic features included a dense, collagenous stroma; prominent, dilated, thin-walled vessels; muscular hyperplasia of small arteries; keloidal change; myxoid change; and fibrous tissue insinuation into the muscularis propria of the bowel. Although mitoses were noted in many tumors, they were usually few in number. The gross and histologic features were similar in the clinical subgroups; however, keloidal change was seen less often in female patients. Less than half of the cases were initially correctly diagnosed. Most patients without Gardner syndrome were without recurrence at follow-up, even when the lesions had been incompletely excised.