Cowden syndrome (CS) is a rare autosomal dominant genodermatosis, characterized by multiple hamartomas, particularly of the skin, associated with high frequencies of breast, thyroid, and genitourinary malignancies. Although Lhermitte-Duclos disease (LDD) or dysplastic gangliocytoma of the cerebellum, a slowly progressive unilateral tumor, is a major criterion of CS, its frequency in patients with CS is unknown. Other cerebral abnormalities, especially meningioma and vascular malformations, have also been described, albeit rarely, in these patients. The aim of the current study was to use cerebral magnetic resonance imaging (MRI) to evaluate LDD frequency and to investigate other brain abnormalities in CS patients recruited by dermatologists. A multicenter study was conducted in 8 hospital dermatology departments between January 2000 and December 2003. Twenty patients with CS were included; specific cerebral MRI abnormalities were found in 35% (7/20) of them. Cerebral MRI revealed LDD in 3 patients, a meningioma in 1, and numerous vascular malformations in 6 patients. Five patients had venous angiomas (3 associated with LDD) and 2 patients had cavernous angiomas (1 associated with LDD and a venous angioma). The discovery of asymptomatic LDD in 3 patients and a cavernous angioma in another prompted us to perform neurologic examinations regularly and MRI to estimate the size and the extension of the tumor, and to assess the need for surgery. CS similarities with Bannayan-Riley-Ruvalcaba (BRR) are discussed because some patients could also have the BRR phenotype (for example, genital lentigines, macrocephaly, multiple lipomas) and because BRR seems to have more central nervous system vascular anomalies. Because CS signs can involve numerous systems, all physicians who might encounter this disease should be aware of its neurologic manifestations. Our findings confirm the contribution of brain MRI to detecting asymptomatic LDD, vascular malformations, and meningiomas in patients with CS.