Huntington's disease is an autosomal dominant neurodegenerative disorder caused by the expansion of a (CAG)n repeat in the IT15 gene. Three per cent of cases are sporadic and in those in which family studies have been performed, the origin of the mutation was always paternal. The first sporadic case of Huntington's disease is presented in which a premutated maternal allele of 37 CAG repeats was transmitted expanded to the proband (43 CAG repeats). Molecular analysis of the IT15 gene is extremely important in sporadic cases of Huntington's disease, providing correct diagnosis of the disorder and facilitating genetic counselling to the family members.