The authors report two patients with familial adenomatous polyposis and thyroiditis. One patient was discovered at autopsy to have in addition, a follicular carcinoma of the thyroid and focal nodular hyperplasia of the liver. The other patient had a sister with familial adenomatous polyposis and a papillary carcinoma of the thyroid. The association between familial adenomatous polyposis and thyroiditis has not been previously reported.