A study of more than 100 families for the first time gauges the size of that risk.
"I'm scared," said Jackie Lustig, 52, of Sudbury, Mass., whose father died of Alzheimer's and whose mother is living it. "I'm hoping to heck that the pharmaceutical companies come up with something better than there is now. It's not a nice way to go."
The study, appearing in March's Archives of Neurology, found more than 22 percent of the adult children of 111 couples with Alzheimer's had the disease themselves. Risk grew with age. Among offspring older than 60, more than 30 percent were affected. In those older than 70, nearly 42 percent had the disease.
Prior studies have found a 6 to 13 percent prevalence of the disease in the U.S. population older than 65.
At age 62, Gayle Dorman worries every time she misplaces her car keys. "Is this the day I'm going to start losing it?" she wonders.
The suburban Tacoma, Wash., woman spent eight years caring for her parents, who died of Alzheimer's, and in a cruel coincidence, her husband's mother, who also died of the illness.
She said she was surprised to learn "a lot of other people have a double whammy like I do." No one knows how many people have two afflicted parents, but experts say that as baby boomers age, there are likely to be more.
For now, there's no cure for the more than 26 million people worldwide estimated to have Alzheimer's, which gradually destroys memory and other mental abilities.
Dorman took part in the University of Washington study to find out more about her risk and to help researchers identify culprit genes that could lead to new treatments. Families were recruited through the university's Alzheimer's research center.
In the study, diagnoses were confirmed through medical records, autopsies and examination by researchers. The parents with Alzheimer's had 297 children who lived to adulthood and 67 of those children had Alzheimer's.
Senior author Dr. Thomas Bird of the University of Washington said he was uncomfortable saying the normal risk tripled or quadrupled in people with two affected parents because the study was small and had no comparison group.
"What I'm comfortable saying is that risk is increased and we're working on trying to find out what the magnitude is," Bird said.
The study was funded by the National Institute on Aging and the U.S. Department of Veterans Affairs.
Bird disclosed in the paper that he has a licensing agreement with Athena Diagnostics Inc. The company does lab testing for a gene related to late-onset Alzheimer's. But Bird's agreement involves genetic discoveries unrelated to Alzheimer's, he said, and the company had nothing to do with the research.
Many people with two affected parents ask their doctors to quantify their risk, experts said.
"I tell them it's our strong hope that by the time they reach the age of risk, we'll have better interventions," said Dr. Steven T. DeKosky of the University of Pittsburgh. He recommends controlling cholesterol and blood pressure, and staying mentally active.
But Dr. David Bennett of Chicago's Rush University Medical Center said evidence is mixed on whether nutrition, exercise and stimulating mental activity can prevent or delay disease in people with culprit genes.
"Lifestyle changes may not be beneficial, but in other cases it may be," Bennett said. "We just need to do the research and figure that out."
Worried about her own risk, Lustig has bought long-term care insurance. She reads up on research and hopes her job will keep her brain active.
"I eat a balanced diet. I exercise," she said.
Would she want to know her exact risk?
"I don't want to know," Lustig said. "I think I've done what I can do. It's sort of in God's hands."
