To clarify the clinical risk of desmoid tumors developing in familial adenomatous polyposis, we reviewed the cases of 49 Japanese patients diagnosed with familial adenomatous polyposis at our institute. In six patients who manifested desmoid tumors at a mean age of 31 years, we reviewed the clinical features and compared various phenotypic manifestations with those in the 43 patients without desmoid tumors. During the observation periods (mean, 6.5 years), two of six patients with desmoid tumors died because of the tumors, which measured >10 cm in diameter at the initial diagnosis, whereas the remaining four patients with desmoid tumors <5 cm did not experience complications. The patients with desmoid tumors tended to be women (5 of 6 vs. 17 of 43; p = 0.05) and more frequently had gastric fundic gland polyposis (5 of 6 vs. 17 of 43; p = 0.05) than did the patients without desmoid tumors. There were no apparent differences in other clinical manifestations, including the incidences of colonic polyposis, gastroduodenal adenomas, and extraintestinal tumors. Desmoid tumors can be a serious complication in patients with familial adenomatous polyposis. There may be some association in the genesis of desmoid tumors and gastric fundic gland polyposis.