The accumulation of oligomeric species of beta-amyloid protein in the brain is considered to be a key factor that causes Alzheimer's disease (AD). However, despite many years of research, the mechanism of neurotoxicity in AD remains obscure. Recent evidence strongly supports the theory that Ca2+ dysregulation is involved in AD. Amyloid proteins have been found to induce Ca2+ influx into neurons, and studies on transgenic mice suggest that this Ca2+ influx may alter neuronal excitability. The identification of a risk factor gene for AD that may be involved in the regulation of Ca2+ homeostasis and recent findings which suggest that presenilins may be involved in the regulation of intracellular Ca2+ stores provide converging lines of evidence that support the idea that Ca2+ dysregulation is a key step in the pathogenesis of AD.