Recent reports of seven cases of Lhermitte-Duclos disease occurring in adult patients with Cowden's syndrome (multiple hamartoma syndrome) strongly suggest that Lhermitte-Duclos disease is one of the types of neoplasia that characterize this syndrome. A case of Lhermitte-Duclos disease is reported in a 16-year-old girl with craniomegaly, choroidal hamartoma, and conjunctival papilloma of the right eye, and a history of bilateral multinodular adenomatous goiter and cystic hygroma. These findings strongly suggest a diagnosis of Cowden's syndrome. Although the syndrome traditionally has been defined by mucocutaneous criteria, it typically also involves hamartomas and neoplasia of internal organs, most commonly in the thyroid, breast, and female genitourinary tract. Because the mucocutaneous features may develop several decades after birth, the present case both supports the previously reported association between Lhermitte-Duclos disease and Cowden's syndrome and highlights the need for long-term follow-up monitoring of a pediatric patient with Lhermitte-Duclos disease because of the risk of malignancies associated with Cowden's syndrome.