Risk of dementia in relatives of patients with Alzheimer's disease.

Using a family history questionnaire, we investigated the occurrence of dementia among relatives of patients with a clinical diagnosis of Alzheimer's disease (AD) and among the relatives of age-matched control subjects. Cumulative lifetime risk of developing AD-type dementia was greater among relatives of AD probands and was consistent with an autosomal dominant genetic mode of transmission. Although the lifetime risk of AD-type dementia was similar among relatives of early-onset and late-onset AD probands, relatives of early-onset probands tended to have an earlier onset of dementia than did relatives of late-onset AD probands. This result raises the possibility that age at onset of dementia in AD may be genetically determined.