A second member of the original family with the valine to isoleucine substitution at codon 717 of the amyloid precursor protein died after the clinical diagnosis of Alzheimer's disease had been made in life. Neuropathological examination of the brain revealed not only severe Alzheimer type pathology, with senile plaques and neurofibrillary tangles, but also Lewy bodies both in the cortex and brainstem. Lewy bodies also occurred in our first case, thus showing striking similarities in these two members of the same family. The possibility exists that the occurrence of Lewy bodies may not be coincidental, but could be genetically determined: the same genetic abnormality which determines the deposition of beta A4 protein, thus triggering of a chain of events leading to Alzheimer's disease, may result in, or predispose to Lewy body formation.