Mutations in the amyloid precursor protein and presenilin 1 and 2 genes result in elevated plasma levels of the amyloid β‐peptide species terminating at amino acid residue 42 (Aβ1–42). In a longitudinal study of unrelated elderly individuals, those who subsequently developed Alzheimer's disease had higher plasma levels of Aβ1–42 at entry than did those who remained free of dementia. The results indicate that elevated plasma levels of the released Aβ peptide Aβ1–42 may be detected several years before the onset of symptoms, supporting that extracellular Aβ1–42 plays an important role in the pathogenesis of late‐onset Alzheimer's disease.