Washington - If both your parents have Alzheimer's disease, you probably are more much likely than other people to get it, researchers said on Monday.

Their study focused on 111 families in which both parents were diagnosed with Alzheimer's disease, the most common form of dementia among the elderly, and assessed the risk for developing it among the offspring.

The parents had 297 children who lived into adulthood. Of the 98 men and women who were at least 70 years old, 41 of them -- about 42 percent -- developed Alzheimer's disease, researchers at the University of Washington in Seattle found.

"That's greater than you would expect in the general population in that age group," Dr. Thomas Bird, one of the researchers, said in a telephone interview.

In the general population, risk for the disease begins to rise at about age 65, with the number of people developing the disease doubling every five years beyond that, experts say.

But about two-thirds of the adult offspring in the study still had not reached age 70. Counting all 297 of these adult offspring regardless of age, 23 percent already had been diagnosed with Alzheimer's disease, with the disease diagnosed on average at age 66, the researchers found.

Bird said that compares to the roughly one in 10 chance that the average person will develop the disease.

"I think it confirms that there's a strong genetic component in the disease and that's not a surprise," said Bird, whose study was published in the Archives of Neurology.

Scientists do not yet fully understand the causes of Alzheimer's disease, although genetics plays an important role. There is no cure.

Bird said there is only one gene, known as ApoE, that is generally agreed among researchers as a risk factor for the disease but there likely are many more.

The ApoE gene is involved in making a substance in the body that helps carry cholesterol in the bloodstream and the gene seems to influence the age of onset of Alzheimer's.

The researchers have been doing the study for about two decades and intend to continue for at least another decade.

"The numbers will be interesting to follow as they get older and older," Bird said.

Bird said the study is not examining the Alzheimer's risk for people who have one but not two parents who develop the disease.

In order to confirm that both parents actually had Alzheimer's, the researchers reviewed the medical records and in many cases the brain autopsies of those who had died, and tried to meet in person to assess those who still living.

In people with Alzheimer's disease, healthy brain tissue degenerates, causing an inexorable decline in memory and mental abilities. The average length of time from diagnosis to death is about eight years.

(Editing by Maggie Fox and Bill Trott)