Multiple hamartoma syndrome (Cowden's disease) consists of characteristic skin lesions of the face, mucous membranes and distal extremities in association with a variety of benign and malignant internal tumors, especially of the thyroid and breast. We describe a family in which the father, daughter and son were found to have goiter associated with the skin lesions of Cowden's disease. Review of the 40 reported cases of this syndrome indicates that thyroid disease occurs in two thirds of patients with Cowden's disease and most often presents as goiter at an early age. Thyroid cancer has occurred in only three (7.5%) of the patients.Surgical removal of the large goiter of the son showed that it was composed of multiple encapsulated follicular adenomas and a few areas of lymphocytic thyroiditis. Studies of the thyroid tissue showed that peroxidase activity was decreased, the thyroglobulin had a reduced content of thyroxine and triiodothyronine (perhaps due to the therapeutic suppression of thyroid-stimulating hormone) and thyroxine 5'-monodeiodinase was greatly increased; increased outer ring monodeiodinase activity may be a characteristic of human follicular adenomas.