Higher whole plasma apoE levels and higher apoE levels in HDL were associated with better cognitive function assessed by ADAS-cog (whole plasma, β coefficient, −0.15; 95% CI, −0.24 to −0.06; HDL, β coefficient, −0.20; 95% CI, −0.30 to −0.10) but were unassociated with dementia or Alzheimer disease risk. This evidence concerns the gene APOE and early-onset autosomal dominant Alzheimer disease.