The brain tissue from a case of familial Alzheimer's disease (FAD) caused by a missense (valine to glycine) mutation at codon 717 of the amyloid precursor protein (APP) gene has been examined for the presence of abnormally phosphorylated paired helical filament tau (PHF-tau). There was abundant PHF-tau present, which on Western blots, was indistinguishable from the PHF-tau typical of cases of sporadic Alzheimer's disease and that of another FAD mutation (valine to isoleucine), previously (Neurosci. Lett., 137 (1992) 221-224). This result implies that the cytoskeletal pathology in Alzheimer's disease is biochemically linked to abnormal APP processing and amyloid deposition.