Aged women appear particularly vulnerable to dementia
Almost half of all women in their 90s are suffering from dementia, Californian research suggests.
The analysis of more than 900 people aged 90 or over, published in the journal Neurology, found it was far less likely in men of the same age.
The reasons are not clear - although older women are more prone to stroke and heart disease, both risk factors for dementia.
There are fears dementia could place a great strain on health services.
There have been few studies looking specifically at dementia in very old people, even though increases in life-expectancy mean that this is a fast-growing group.
Other studies have shown that dementia prevalence increases for both men and women between the ages of 65 and 85.
However, the Californian research found that the likelihood of having dementia doubled every five years in women after reaching 90, but not in men.
A total of 45% of the women had dementia, compared with 28% of men. It also suggested that women who had received higher education were much less likely to develop dementia than those with a lower level of education.
Dr Maria Corrada, who led the study, said: "As more and more people reach age 90, our findings provide further evidence that more needs to be done to provide adequate resources to care for the increasing number of very old people with memory problems."
A recent report by the King's Fund suggested that the burden of dementia in the UK was likely to rise sharply over the next two decades as the population aged.
The total bill for care, it predicted, would more than double to over £35 billion a year, as the number of people with the illness rises past 900,000.
The Alzheimer's Society said, with this in mind, there was now an urgent need to find out more about how gender affected the likelihood of dementia.
A spokesman said: "Previous research has suggested that, as they get older, women are more prone to stroke and heart disease.
"Both of these are risk factors for dementia so this may go some way in explaining the difference."