Alzheimer's disease (AD) is characterized by the deposition of senile plaques (SPs) and neurofibrillary tangles (NFTs) in vulnerable brain regions. SPs are composed of aggregated β-amyloid (Aβ) 40/42(43) peptides. Evidence implicates a central role for Aβ in the pathophysiology of AD. Mutations in βAPP and presenilin 1 (PS1) lead to elevated secretion of Aβ, especially the more amyloidogenic Aβ42. Immunohistochemical studies have also emphasized the importance of Aβ42 in initiating plaque pathology. Cell biological studies have demonstrated that Aβ is generated intracellularly. Recently, endogenous Aβ42 staining was demonstrated within cultured neurons by confocal immunofluorescence microscopy and within neurons of PS1 mutant transgenic mice. A central question about the role of Aβ in disease concerns whether extracellular Aβ deposition or intracellular Aβ accumulation initiates the disease process. Here we report that human neurons in AD-vulnerable brain regions specifically accumulate γ-cleaved Aβ42 and suggest that this intraneuronal Aβ42 immunoreactivity appears to precede both NFT and Aβ plaque deposition. This study suggests that intracellular Aβ42 accumulation is an early event in neuronal dysfunction and that preventing intraneuronal Aβ42 aggregation may be an important therapeutic direction for the treatment of AD.