Women who have ovaries removed before the menopause could be increasing their risk of both Parkinson's Disease and memory problems, research suggests.
The female hormone oestrogen may protect the brain
Removing one or both ovaries at a young age doubled the risk.
Sex hormones made by the ovary may be protecting the brain, said experts from the US Mayo Clinic.
A Parkinson's Disease Society spokesman said the Neurology journal study might explain why more men developed the disease.
A large number of women each year have an ovary removed as a result of a cyst, ovarian cancer, or endometriosis.
The health of thousands of women who had undergone surgery on average 27 years ago was examined by the Mayo Clinic researchers.
As well as checking records for diagnoses of Parkinsonism - movement disorders such as Parkinson's Disease, which can cause uncontrollable muscle tremors - they questioned either the woman or her relatives about the state of her memory.
They found that ovary removal prior to the menopause was clearly linked to an increased chance of poor memory and even dementia.
In addition, Parkinsonism was much more likely in women who had had the operation before the menopause.
One of the main roles of the ovary, aside from the storage and delivery of eggs, is to produce oestrogen, one of the most important female sex hormones.
Hormone replacement therapy is given to some women who have one or both ovaries removed, but the majority either do not receive it at all, or only get it after the age of 50, said the researchers.
Dr Walter Rocca, the lead researcher, said: "It's possible that oestrogen has a protective effect on the brain and that a lack of oestrogen due to ovary removal may increase a woman's risk of developing memory problems."
He called on doctors to think carefully about the consequences of ovary removal in younger women.
Dr Kieran Breen, Director of Research and Development for the Parkinson's Disease Society, said: "It is well known that oestrogen can protect nerve cells against the effects of external toxins that may be associated with the nerve cell death that gives rise to diseases such as Parkinson's and Alzheimer's.
"This study reinforces previous findings in this area. This also helps to explain why Parkinson's is more prevalent in men than in women."