We found a new familial Alzheimer's disease kindred in which the disease cosegregates with the APP717Val-->Ile mutation and in which all of the three most common apolipoprotein E (ApoE) alleles are represented. We studied the relationship between ApoE genotype and the clinical expression of the disease and found that in this amyloid precursor protein-mutated family, ApoE genotype influences the age at onset of the disease. Three mutated subjects heterozygous for the epsilon 4 allele had the earliest age at onset in this family, subjects heterozygous for the epsilon 2 allele had the latest age at onset, and subjects homozygous for the epsilon 3 allele had an intermediate age at onset. In this large kindred we also found an amyloid precursor protein-mutated subject 2.4 standard deviations older than the mean age at onset without clinical signs and symptoms of the disease and carrying the epsilon 2/epsilon 3 genotype.