A Lhermitte-Duclos type cerebellum hamartoma is reported in a woman with a diffuse hamartomatous condition involving the breast, thyroid, colon, skin, and kidney. The family history demonstrated the autosomal dominant transmission of this hamartomatous syndrome, and indicated the diagnosis of Cowden disease. This observation supports the recent hypothesis of Lhermitte-Duclos disease associated with Cowden disease as being a single phakomatosis.