Identical twins discordant for presenile dementia of the Alzheimer type.

In genetically proven identical female twins, discordant for presenile dementia of the Alzheimer type, the affected twin began to dement at the age of 49, and died 15 years later; the diagnosis was confirmed at post-mortem. The surviving twin remains clinically unaffected 20 years after the onset of dementia in her sister. Environmental aetiological factors are postulated to account for this discordance.