Compared with APOE‐ε4 negativity, both APOE‐ε4 heterozygosity (β = 10.498, T = 2.608, p = 0.00931) and APOE‐ε4 homozygosity (β = 17.283, T = 2.316, p = 0.02086) were associated with steeper decline in CSF Aβ1‐42 on approach of age of parental dementia onset. This evidence concerns the gene APOE and dementia.