A case of intra-abdominal desmoid tumors in familial polyposis coli (FPC), which regressed and disappeared by prednisolone treatment, is reported. A 37-year-old Japanese man with abdominal lumps was admitted to our hospital. He had had proctocolectomy two years before because of FPC with rectal cancer. At laparotomy, tumors were present in the abdominal wall, mesentery, and retroperitoneum. Only a small part of the tumors was resected and diagnosed microscopically to be desmoid tumors. With prednisolone administration (20 to 5 mg/day) subjective symptoms were ameliorated and desmoid tumors slowly regressed. Bilateral hydronephrosis continued and resulted in "retroperitoneal fibrosis." To our knowledge, this case is the first well-documented case of retroperitoneal fibrosis in a patient with FPC. The characteristics of the desmoid tumor in familial polyposis coli or in Gardner's syndrome and the methods for its management are discussed.