Two unrelated patients with macrocephaly, seizures, and mild cerebellar signs had a dysplastic gangliocytoma of the cerebellum (Lhermitte-Duclos disease). Both also had autosomal dominant Cowden disease as evidenced by facial, oral, and acral papules. In the two families, 9 sibs demonstrated the mucocutaneous lesions, thyroid disease, breast tumors, and ovarian tumors compatible with the diagnosis of Cowden disease. Some of the sibs also showed various degrees of neurological signs such as macrocephaly, mental retardation, seizures, tremor, and dysdiadochokinesia. Magnetic resonance imaging scans of sibs of one family demonstrated megalencephaly and other mild abnormalities. The occurrence of these two rare disorders in single patients is more than a coincidence, and the clinical findings in the combined condition establishes it as a new phakomatosis.