Dementia of the Alzheimer type: clinical and family study of 22 twin pairs.

We studied 22 twin pairs in which one or both twins had dementia of the Alzheimer type (DAT). In four twins, diagnosis was confirmed by autopsy. Seven monozygotic (MZ) pairs were concordant for DAT; 10 MZ pairs were discordant. Two dizygotic (DZ) pairs were concordant for DAT, and 3 DZ pairs were discordant. The current concordance rate was 41% for MZ twins and 40% for DZ twins. The study supports the belief that, etiologically, DAT cannot be entirely accounted for by a single autosomal dominant gene. The data also suggest that in certain genetic circumstances, disease expression may be delayed in females.