Cowden's disease, also known as various hamartomatous malformations of multiple organs, is a rare autosomal dominant disorder. The most important feature of Cowden's disease is its frequent association with malignant neoplasm, particularly in the breast and thyroid gland. Cowden's disease with malignant neoplasms is quite rare in Korea so far. We report a case of Cowden's disease associated with breast cancer in a 41-year-old female who underwent routine health cheek-up.