Lhermitte-Duclos disease, or dysplastic gangliocytoma of the cerebellum, is a peculiar hamartoma arising from the cerebellar cortex. Cowden disease, which is also termed as a multiple hamartoma-neoplasia syndrome, is an unusual autosomal dominant disorder characterized by mucocutaneous lesions, including facial papules, gingival papillomas, and acral keratoses. Other systemic hamartomas are frequently present, and there are high incidence of breast, thyroid, and genitourinary malignancies. Recently, it is accepted that Lhermitte-Duclos disease is a part of Cowden disease, a new phakomatosis. Recognition of this association has clinical significance, because diligent long-term follow-up monitoring of individuals with Lhermitte-Duclos disease and Cowden disease may lead to the early detection of malignancy.