Cowden's syndrome (CS), or multiple hamartoma syndrome, is an autosomal dominant disorder associated with benign skin tumors and an increased risk of breast cancer. In an effort to understand the basic mechanisms regulating the development of breast cancer in this patient population, as well as to define diagnostic aspects of the disorder, we describe for the first time the clinical and pathological spectrum of breast disease in CS. We obtained the clinical histories and examined the histopathology of 59 cases from 19 women with CS sent to us from a variety of institutions. The 19 women showed a spectrum of benign histopathological findings, including ductal hyperplasia, intraductal papillomatosis, adenosis, lobular atrophy, fibroadenomas, and fibrocystic change. Seventeen (89%) showed features suggestive of a breast hamartoma. Fourteen women (74%) showed malignant disease, most of which was ductal carcinoma. Twelve patients (86%) showed ductal carcinoma in situ (DCIS), and 12 (86%) showed infiltrating ductal carcinoma. One patient had only DCIS and another patient showed both infiltrating tubular carcinoma and lobular carcinoma in situ. Ten patients (71%) actually showed foci of tumor involving densely fibrotic, hamartomatous areas. In summary, we show that women with CS have a spectrum of exuberant benign and malignant breast pathology. A common benign breast lesion in CS is a densely fibrotic hyalinized nodule, whereas the most frequent breast malignancy is ductal carcinoma.