The deposition of aggregated amyloid beta-protein (Abeta) in the human brain is a major lesion in Alzheimer' disease (AD). The process of Abeta fibril formation is associated with a cascade of neuropathogenic events that induces brain neurodegeneration leading to the cognitive and behavioral decline characteristic of AD. Although a detailed knowledge of Abeta assembly is crucial for the development of new therapeutic approaches, our understanding of the molecular mechanisms underlying the initiation of Abeta fibril formation remains very incomplete. The genetic defects responsible for familial AD influence fibrillogenesis. In a majority of familial cases determined by amyloid precursor protein (APP) and presenilin (PS) mutations, a significant overproduction of Abeta and an increase in the Abeta42/Abeta40 ratio are observed. Recently, it was shown that the two main alloforms of Abeta have distinct biological activity and behaviour at the earliest stage of assembly. In vitro studies demonstrated that Abeta42 monomers, but not Abeta40, form initial and minimal structures (pentamer/hexamer units called paranuclei) that can oligomerize to larger forms. It is now apparent that Abeta oligomers and protofibrils are more neurotoxic than mature Abeta fibrils or amyloid plaques. The neurotoxicity of the prefibrillar aggregates appears to result from their ability to impair fundamental cellular processes by interacting with the cellular membrane, causing oxidative stress and increasing free Ca(2+) that eventually lead to apoptotic cell death.