Hormone replacement therapy (HRT) may protect post-menopausal women against memory loss and Alzheimer's disease.
There is no cure for dementia
A study found women's memories are affected when their bodies stop producing the hormone oestrogen - as happens at the menopause.
However, the Institute of Psychiatry at King's College London found memory recovered when hormone supplies were restored - the effect achieved by HRT.
Women have a higher risk of developing Alzheimer's than men.
It is estimated that around 450,000 women in the UK have the disease.
There is a theory that oestrogen may help prevent the build up of damaging protein tangles in the brain which are thought to trigger cell death, and Alzheimer's.
But when supplies of the hormone are abruptly cut at the menopause, women may become more vulnerable.
A major US study found no evidence of a protective effect from HRT - and even suggested the combined form of the treatment might increase the general risk of dementia.
The trial was halted early when when results suggested some women taking HRT had an increased risk for breast cancer and cardiovascular disease.
However, UK experts argue that this study focused on older women, who did not receive HRT immediately after the menopause.
The latest research was carried out on 30 young, pre-menopausal women who were having surgery for benign womb growths called fibroids.
The patients were given a drug which shrinks the fibroids, but which also temporarily shuts down the ovaries, producing a state similar to the menopause.
Each woman was given a memory test before, during and after treatment, when their ovaries were once again producing oestrogen.
Average scores were similar before and after treatment, but dipped by about 10% while the women's ovaries were shut down.
The researchers also recorded reduced levels of activity in an area of the brain called the pre-frontal cortex during treatment, which is linked to learning and memory.
Lead researcher Dr Michael Craig said evidence was growing that sex hormones, such as oestrogen, did have a direct effect on the brain.
He said: "There may be a critical window of time around the menopause when HRT may have a beneficial effect in protecting against Alzheimer's dementia.
The Alzheimer's Society said HRT could not be viewed as a treatment option for reducing risk or delaying the onset of Alzheimer's disease.
It said although some previous research had also linked HRT to a lower risk of Alzheimer's, this could be because women taking HRT were also doing other things to preserve their health, and reduce their risk.
Professor Clive Ballard, the society's director of research, said: "Using HRT as a treatment or preventative measure against Alzheimer's disease would mean giving drugs, which can produce side-effects, to women without symptoms."
Rebecca Wood, chief executive of the Alzheimer's Research Trust, said: "This is not conclusive evidence and women are not advised to start HRT specifically to protect against dementia since it can have side-effects and possibly increase the risk of stroke."