Women who have their ovaries removed may be at increased risk of dementia, a study of 2,500 patients suggests.
A Mayo Clinic, Minnesota, team found the risk was especially raised among young women - removal of one ovary by the age of 38 raised the risk by 260%.
They believe the key could be loss of the female sex hormone oestrogen, which may help prevent ageing.
Alternatively, they said women at risk of ovarian cancer may also be genetically at risk of dementia.
The findings were presented to an American Academy of Neurology meeting.
The researchers studied 1,209 women who had both ovaries removed, and 1,302 who had one removed, between 1950 and 1987.
They were compared over time with a similar number of woman who had not undergone surgery.
Overall, women who had undergone surgery were 40% more likely to develop signs of dementia or cognitive impairment than those in the healthy control group.
Women who had both ovaries removed by the age of 46 had a 70% increased risk of dementia or cognitive impairment.
And those who had one ovary removed before the age of 38 had a 260% increased risk.
Lead researcher Dr Walter Rocca said women who have both ovaries removed received oestrogen replacement therapy (HRT).
But he said it was possible the treatment was not continued for long enough, particularly if surgery was carried out at a young age.
Dr Rocca said the latest findings may have an implication for women who opt for ovary removal to reduce the risk of developing cancer later in life.
"Like any medical or surgical decision, there is a trade between risk and benefit.
"Our findings are important for situations where the removal of the ovaries is elective - that is, conducted to reduce the risk of ovarian cancer."
Dr Rocca's team has previously found that ovary removal increases the risk of Parkinson's disease.
The surgery has also been linked to an increased risk of cardiovascular disease and the bone disease osteoporosis.
Henry Scowcroft, of the charity Cancer Research UK, said: "Removing the ovaries is only done when absolutely necessary.
"It is important to bear this in mind when considering the theoretical risk of possible future problems.
"These results have not yet been published in a peer-reviewed journal so it is difficult to comment on the relative risks involved."
Dr Susanne Sorensen, head of research at the Alzheimer's Society, said it had long been thought that oestrogen might have a protective effect against the development of dementia.
However, she said a large international study had found no evidence that HRT had any beneficial effect.
She said: "These results again seem to indicate that there may be a link between the lack of oestrogen and the development of dementia, although the researchers state that the link may be more complex than thought earlier."