In hereditary cerebral hemorrhage with amyloidosis, Dutch type (HCHWA-D), a genetic variant (E22Q) of amyloid  (A) accumulates predominantly in the small vessels of leptomeninges and cerebral cortex, leading to fatal strokes in the fifth or sixth decade of life. A deposition in the neuropil occurs mainly in the form of preamyloid, Congo red negative deposits, while mature neuritic plaques and neurofibrillary tangles, hallmark lesions in Alzheimer's disease (AD), are characteristically absent. A recent hypothesis regarding the pathogenesis of AD states that A extending to residues 42-43 (as opposed to shorter species) can seed amyloid formation and trigger the development of neuritic plaques followed by neuronal damage in AD. We characterized biochemically and immunohistochemically A from three cases of HCHWA-D to determine its length in vascular and parenchymal deposits. Mass spectrometry of formic acid-soluble amyloid, purified by size-exclusion gel chromatography, showed that A 1-40 and its carboxyl-terminal truncated derivatives were the predominant forms in leptomeningeal and cortical vessels. A 1-42 was a minor component in these amyloid extracts. Immunohistochemistry with antibodies S40 and S42, specific for A ending at Val-40 or Ala-42, respectively, were consistent with the biochemical data from vascular amyloid. In addition, parenchymal preamyloid lesions were specifically stained with S42 and were not labeled by S40, in agreement with the pattern reported for AD, Down's syndrome, and aged dogs. Our results suggest that in HCHWA-D the carboxyl-terminal A heterogeneity is due to limited proteolysis in vivo. Moreover, they suggest that A species ending at Ala-42 may not be critical for the seeding of amyloid formation and the development of AD-like neuritic changes.Hereditary cerebral hemorrhage with amyloidosis, Dutch type (HCHWA-D), 1 is an autosomal dominant form of cerebral amyloidosis clinically defined by recurrent strokes and fatal cerebral bleeding in the fifth or sixth decade of life (1). Amyloid deposits are preferentially localized in the walls of small arteries of the leptomeninges and cerebral cortex. Parenchymal deposition is largely in the form of preamyloid lesions or diffuse plaque-like structures that are Congo red-negative and lack the dense amyloid cores commonly present in Alzheimer's disease (AD). Immature or early plaques are occasionally found surrounded by dystrophic neurites, although neurofibrillary tangles are consistently absent (2).Biochemically, HCHWA-D amyloid fibrils are composed of a genetic variant of amyloid  (A) of AD and Down's syndrome (3, 4) in which there is a glutamine (Gln) for glutamic acid (Glu) at position 22 due to a missense mutation (cytosine for guanine) at codon 693 of the  precursor protein (PP) 770 gene (5). This mutation is pathogenic since it segregates with the disease (6). Several in vitro studies have shown that Glu 3 Gln substitution yields an A variant that has an abnormal conformation (higher content of -sheet), forms amyloid-lik...