Major research suggests hormone replacement therapy may increase the risk of developing dementia.
The women in the study took HRT tablets
It had been thought that HRT could be used as a treatment to delay or prevent the onset of dementia.
But the US Women's Health Initiative study in the Journal of the American Medical Association found oestrogen-only HRT may increase dementia risk.
Previous findings from the group had showed an increased risk from taking combined HRT.
Around 1.5m women in the UK take HRT to relieve unpleasant menopausal symptoms, with half taking the oestrogen-only therapy, which is used by women who have undergone a hysterectomy.
But experts had also hoped HRT could be used to delay or prevent dementia - because oestrogen had been shown to increase blood flow to the brain.
The researchers from Wake Forest University Baptist Medical Center involved in the Women's Health Initiative Memory Study assigned around 3,000 women aged between 65 and 79 to either take a daily tablet of oestrogen, or a dummy version, which would have no effect.
Another 4,500 women had either been given combined oestrogen/progestin HRT or a dummy version.
None had any symptoms of dementia when the study began in 1995.
It was found that, when the findings for the two groups were combined, women on either form of HRT had a 76% greater risk of developing dementia, compared to women who were taking dummy pills.
The researchers stressed that while percentage was high, the numbers who developed dementia in either group were relatively small.
However, they say the increase in risk is significant because of the hopes that HRT could be used as a treatment for dementia.
Stephen Rapp, professor of psychiatry and behavioural medicine and one of the lead authors on the study, told BBC News Online: "Our findings show quite clearly that HRT should not be used to protect against the risk of dementia.
"And for some women, there is an increased risk of developing dementia with hormonal therapy."
Professor Peter Bowen-Simpkins of the Royal College of Obstetricians and Gynaecologists, said: "These findings are low in terms of numbers, but they are directly contrary to what we previously thought.
"It's disappointing for the people who thought HRT could delay or prevent dementia."
He said more research was needed to see if larger studies reached the same conclusions, and the effect if women continued taking HRT from the onset of menopause rather than beginning in their early 60s.
The analysis of HRT's effect on memory is one of a range of studies by the Women's Health Initiative looking at how the therapy affects a range of conditions.
In July 2002, women involved in WHI studies were told to stop taking combined HRT because the risks of developing breast cancer, strokes and cardiovascular disease outweighed the benefits.
In February 2004, women in the oestrogen-only study were told to stop taking their drugs due to an increased risk of stroke, and no benefit for heart disease.
All those who took part in the studies will be followed to assess the effects of hormone therapy once treatment is stopped.