['Plasma A beta levels are elevated in early-onset Alzheimer disease (AD) caused by autosomal dominant mutations. Our objective was to determine whether similar genetic elevations exist in late-onset AD (LOAD).', 'We measured plasma A beta in first-degree relatives of patients with LOAD in a cross-sectional series and in extended LOAD families. We screened these subjects for pathogenic mutations in early-onset AD genes and determined their ApoE genotypes.', 'Plasma A beta is significantly elevated in the LOAD first-degree relatives in comparison to unrelated controls and married-in spouses. These elevations are not due to ApoE epsilon 4 or pathogenic coding mutations in the known early-onset AD genes.'] Is plasma amyloid beta protein elevated in late-onset Alzheimer disease families?