We found a novel polymorphism in the amyloid precursor protein (APP) gene in a patient with ischemic cerebrovascular disease who had no evidence of Alzheimer's disease (AD). This polymorphism deletes a Fok I restriction enzyme site and causes the substitution of threonine for alanine at codon 673. This is adjacent to the site at which APP is thought to undergo cleavage in AD. Analysis of this polymorphism may provide insight into the basis of APP processing.