The prevalence of clinical signs and neuropathological findings of Alzheimer's disease (AD) is high in Down's syndrome (DS). In the general population, the apolipoprotein E (ApoE) ϵ4 isoform is an important risk Marchker for AD. We studied the allelic frequencies of ApoE in 26 DS cases fulfilling clinical diagnostic criteria for AD and in 26 DS controls matched for age, sex, and premorbid level of mental retardation. A meta-analysis of data available in the literature was used for comparison with allele frequencies in other AD and control populations. ApoE type 2, 3, or 4 allele freqeuencies were not significantly different in AD–DS cases and DS controls. The ApoE ϵ4 frequency in DS cases with AD (0.14; CI, 0.06–0.26) was significantly lower than in any other AD population studied so far and it is within the range of nondemented controls from the general population. These findings suggest that ApoE ϵ4 does not significantly affect the pathogenesis of AD in DS patients.