An HRT trial has been stopped in its tracks after researchers found the drug could increase the risk of stroke.
Oestrogen-only HRT has been linked to an increased rate of stroke.
The study, part of the Women's Health Initiative run by the National Institutes of Health, saw 11,000 women taking an oestrogen-only pill.
Another branch of the same trial, which tested a combination pill was stopped in 2002 over similar concerns.
Around one quarter of women in the UK who are on HRT take this form of the drug.
The latest study had been running for seven years and tested out Wyeth Co.'s Premarin, which contains 0.625mg of oestrogen.
This would be considered a high dosage for a woman in the over 50 age bracket in the UK.
Although analysis of the data on the latest study has not yet been finalised, researchers say it appears the increased risk of stroke is similar to the previous combination-pill study.
This saw an extra eight strokes a year per 10,000 women, an increase from 21 to 29.
The NIH today issued a statement to say it believed an "increased risk of stroke was not acceptable in healthy women in a research study."
'No link to breast cancer'
However the latest study also found that this type of HRT did not affect heart disease or breast cancer, and actually decreased the risk of hip fractures.
A UK study last year of one million women found an increased risk of breast cancer among women taking a combined HRT pill.
Professor David Purdie, an expert on HRT and osteoporosis in Edinburgh, said the latest findings were welcome news.
"Here is the first major study to show there is no link between HRT and breast cancer," he told BBC News Online.
He said the risk of suffering stroke from taking HRT is still tiny.
Professor Purdie said this study could encourage women to take oestrogen-only HRT.
At the moment the only women on this type of therapy are those who have had hysterectomies.
The previous study run through the Women's Health Initiative involved a pill that combined oestrogen and progestin.
It was called off in July 2002 after researchers found it increased the risk of breast cancer by 26% and heart attack by 29%.
A UK study was stopped in October 2002 after a trial found it could cause a tiny increase in the incidence of heart disease.
Originally experts believed HRT could help prevent heart disease, osteoporosis and memory loss.
However HRT has now been linked to a wide range of conditions including Alzheimer's disease, asthma and age-related deafness.
The UK Department of Health said it would be carefully considering the study findings and information on HRT would be updated as necessary.
It said women should discuss the known risks and benefits of HRT with their doctor and have regular check-ups to assess their need to continue treatment.