Abstract βA4 protein immunoreactivity in the neocortex and hippocampus of familial Alzheimer's disease (AD) including the case with the βA4 amyloid precursor protein (APP) gene mutation in codon 717 (APP717 Val→Ile) and sporadic cases of AD is described. A semi-automatic image analysis system was used to quantify βA4 protein load in the isocortex of the frontal and temporal lobes and in subfields of the hippocampus. Immunoreactivity was measured in ten cases of sporadic AD and in five cases of familial AD including one in which the APP717 Val→Ile mutation was present. βA4 protein load, as measured by square microns of immunoreactivity per square millimetre of cortex, was similar in the frontal and temporal isocortex in both sporadic and familial AD. There was greater variation in βA4 protein load in subfields of the hippocampus but these differences were not significant between sporadic and familial cases. In the case with the APP717 Val→Ile mutation, βA4 protein load in isocortex was greater than the mean for familial and sporadic cases of AD but less than the most severe cases of βA4 protein deposition which were found in sporadic AD. In addition, the case with the APP717 Val→Ile mutation has the same cytoskeletal pathology as sporadic cases of AD. The mechanism by which normal and mutant APP is processed to produce amyloidogenic fragments remains to be determined.