Presenilin 1 mutations are the major cause of autosomal dominant Alzheimer's disease: here we identify a new missense mutation causing a methionine to valine change at codon 233. This codon is homologous to a pathogenic presenilin 2 mutation with the same base change (ATG to GTG) and amino acid change (M239V). This mutation causes disease with an exceptionally early onset age (approximately 30 years) in which pathological examination shows extensive Lewy bodies as well as plaques and tangles.