Plasma Abeta42 (amyloid beta42 peptide) is invariably elevated in early-onset familial Alzheimer's disease (AD), and it is also increased in the first-degree relatives of patients with typical late-onset AD (LOAD). To detect LOAD loci that increase Abeta42, we used plasma Abeta42 as a surrogate trait and performed linkage analysis on extended AD pedigrees identified through a LOAD patient with extremely high plasma Abeta. Here, we report linkage to chromosome 10 with a maximal lod score of 3.93 at 81 centimorgans close to D10S1225. Remarkably, linkage to the same region was obtained independently in a genome-wide screen of LOAD sibling pairs. These results provide strong evidence for a novel LOAD locus on chromosome 10 that acts to increase Abeta.