Some cases of spongiform encephalopathies are linked to mutations within the prion protein gene (PRNP). Repetitive octapeptide insertions of variable length in the PRNP gene are also associated with spongiform encephalopathies, mostly familial Creutzfeldt-Jakob disease (CJD). In this study we report on a novel insertion mutation comprising nine extra octapeptide repeats between codons 51 and 91 of the PRNP gene. The affected patient showed a slowly progressive dementia of at least 6 years duration and ataxia.