Alzheimer's disease. Morbid risk among first-degree relatives approximates 50% by 90 years of age.

The morbid risk of Alzheimer's disease was studied in first-degree relatives of 50 patients who met contemporary clinical research diagnostic criteria and 45 matched controls. Relatives of patients showed a 46% cumulative incidence of probable Alzheimer's disease by 86 years of age. The risk, which was four times the control value, is consistent with other recent reports using similar, modern methods. Although not conclusive, the data suggest the operation of a relatively common, dominant autosomal gene for Alzheimer's disease, the expression of which is delayed until late old age but is largely complete by 90 years of age.