Creutzfeldt-Jakob disease (CJD) is a rapid progressive mental and neurological disorder characterised by dementia and is both infectious and genetic. Pathogenic mutations and a predisposing polymorphism have been described in the prion protein gene and an abnormal prion product accumulates in the brain of affected patients. Apolipoprotein E (APOE), a protein of lipid metabolism, has been detected in some prion protein deposits. This ApoE exists as three common isoforms, coded by specific allele (epsilon 2, epsilon 3, epsilon 4). The presence of at least one epsilon 4 allele was described as a major risk factor for Alzheimer's disease, another neurodegenerative disorder. From a series of 61 patients with CJD we found that epsilon 4 allele of the APOE gene was a risk factor for the disease (p < 0.01). This association was observed in both definite and probable cases, and for patients with and without prion protein gene mutations. Moreover, in affected subjects, epsilon 2 allele of the APOE gene delayed occurrence of death (p < 0.01) independently of other known mutations influencing the phenotype of the disease. These effects on neurodegenerative disease associated with APOE alleles suggest a strong involvement of the APOE locus in brain metabolism.