Dr. Suman Jayadev of the University of Washington in Seattle and colleagues studied the frequency of Alzheimer's disease in adult children of 111 families in which both parents had been clinically diagnosed with the disease. Ages at onset of dementia also were noted.

The study, published in the Archives of Neurology, found of the 297 offspring who reached adulthood, 22.6 percent developed Alzheimer's disease compared with an estimated 6 percent to 13 percent of the general population.

The average age at onset for children of couples with the illness was 66.3. The risk of developing the disease increased with age with 31 percent of those older than age 60 affected and 41.8 percent of those older than age 70 affected.

"Of the 240 unaffected individuals, 78.8 percent had not yet reached age 70 years, suggesting that the incidence of Alzheimer's disease -- 22.6 percent -- is an underestimation of the final incidence rate of Alzheimer's disease in this population," the authors said in a statement.