Alzheimer's disease (AD), which accounts for approximately 50 per cent of autopsied cases of dementia, is a serious, prevalent, and debilitating disease that has ramifications throughout the family and the community. Although this form of dementia is a surprisingly common disorder, destroying certain vital cells of the brain, its aetiology and pathogenesis remain undertermined. However genetic, viral, immunological, and toxic factors have been suggested as influential. Also, it remains unclear whether AD should be considered a single disease, two major diseases ('presenile' and 'senile' AD) or a heterogeneous group of disorders having similar clinical and pathological characteristics, more recent neuroanatomical and biochemical studies providing growing support for clinical and pathological heterogeneity in this so-called primary degenerative 'disease' of the brain.