Alzheimer's disease (AD) and frontotemporal dementia (FTD) are characterized by progressive neuronal loss and microvacuolization, although with different distributions of cortical involvement. In contrast to AD there is no amyloid, senile plaques or tangles in FTD. The involvement of chromosome 19 in AD has been associated with apoliprotein E (ApoE) and the epsilon 4 gene frequency has been related to increased risk and early onset of AD. Our analysis of frequency of the ApoE alleles in 38 patients with AD, 21 patients with FTD and 29 normal controls indicates an association of both AD and FTD with an increased frequency of the epsilon 4 allele and in AD also with homozygosity for epsilon 4. Our results might indicate that ApoE epsilon 4 is an important aggravating and pathoplastic factor in the presence of genetic and other determinants for the development of AD or FTD. A significantly higher epsilon 2 frequency in our FTD material compared to AD and normals might also indicate a connection with the distribution of cortical degeneration.