Those who also stand to lose the most cognitively - women over 85 - appear to gain the most from HRT exposure

Dr Michelle Carlson, of Johns Hopkins University

How people are affected ranges from virtually no change at all, through impaired memory to forms of dementia, of which the most common is Alzheimer's disease.

Previous studies looking at the effects of HRT on slowing down the rate of decline have been mixed, with some showing HRT does have beneficial effects, and others showing none.

Monitoring

The US study looked at over 2,000 women, aged 65 and over.

Information about the women's social and occupational status, their psychiatric and medical history was recorded.

Whether they had a healthy lifestyle was assessed by asking if they used multivitamins and calcium supplements.

The women, all from Cache County, Utah, were followed for three years, and monitored for changes in thinking, learning, depression - and their use of HRT.

They were asked if they were using HRT, had used it in the past or had never used it.

Older women as well as women currently experiencing the menopause might find a period of HRT use beneficial

Dr Linda Clare, University College London

Women who were younger and less depressed, who had more years of education, and who had a particular gene variant (Apo-e4) known to be related to a higher risk of developing Alzheimers disease, scored better initially on the test of cognitive functioning.

Allowing for these effects, it was found that HRT use was linked to better cognitive scores at the first assessment and with less decline in scores by the time of the follow up assessment at the end of the study.

How the hormone treatment did this was not clear but researchers said HRT could make a genuine difference to women's lives.

How long HRT was used for, or whether the women were still using it did not seem to affect results.

Dr Michelle Carlson, of Johns Hopkins University, said the study showed an apparent benefit of taking HRT on the cognitive function in non-demented older women.

She added: "And those who also stand to lose the most cognitively - women over 85 - appear to gain the most from HRT exposure."

But the researchers say more work needs to be done to confirm their conclusions because of the unusual nature of their study group.

All the women live in a geographically-isolated close-knit community.

Ninety per cent are members of the Church of Jesus Christ of the Latter-Day Saints, which prohibits alcohol and tobacco use, which could mean the women are healthier than an average population.

Dr Carlson added: "Though our promising analyses controlled for several confounding factors, our results cannot be regarded as conclusive."

Further studies looking at the effect of HRT on Alzheimer's and cognitive decline are underway.

Protection

A lecturer in clinical psychology at University College London, Dr Linda Clare, told BBC News Online she welcomed the Cache County study as an important contribution to the area of research.

But she said it would need to be backed up by clinical trials.

Dr Clare, who specialises in cognitive impairment in dementia, added: "Older women as well as women currently experiencing the menopause might find a period of HRT use beneficial."

But she warned: "HRT may be protective for some, but not everyone will benefit."