Lhermitte-Duclos disease is a rare disorder of the cerebellum which exhibits a focally indolent growth of the cerebellar cortex. The disease is sometimes associated with multiple hamartoma syndrome, or Cowden disease, an autosomal, dominantly inherited cancer syndrome. Germline mutations of the PTEN/MMAC1 gene, a gene recently discovered to be a tumor suppressor, have been documented in four families with Cowden disease. A family member in one of these families have had Lhermitte-Duclos disease, indicating that mutations of the gene are responsible for the development of Lhermitte-Duclos disease. However, the occurrence of Lhermitte-Duclos disease is mostly sporadic. It is unknown whether the PTEN/MMAC1 gene is involved in these cases. We performed a genetic analysis on a patient with Lhermitte-Duclos disease, as well as analyses on his family members, and found a germline mutation of the PTEN/MMAC1 gene. An insertion of A at nucleotide 83 in codon 28 was apparent in both the patient and members of his family. This mutation caused a frame shift that generated a premature stop codon in codon 43. The mutation was heterozygous, although only the mutated allele was expressed in the cerebellar tumor of the patient. A monoallelic expression of the mutated PTEN/MMAC1 gene may have been responsible for the development of a cerebellar tumor in the patient.