However, they found little evidence that it has any affect on other more common ones.

Although HRT appears to have a protective effect against heart disease and osteoporosis, some fear it increases the risk of some types of breast cancer.

That it only seems to affect the risk of a relatively uncommon type that is easily treatable is reassuring, doctors have said.

However, the researchers have called for more research to establish the exact relationship between HRT and other types of breast cancer.

Establishing a connection

Three doctors - Susan Gapstur and Monica Morrow of Northwestern University, Chicago, and Thomas Sellers, of the Mayo Clinic, Minnesota - studied more than 37,100 postmenopausal women in the Iowa Women's Health Study.

They wanted to find out if hormone replacement therapy was associated with subsequent risk for specific types of breast cancer.

They found there was a slight increase in an invasive - meaning it spreads - cancers and that the risk got greater the longer the woman took HRT.

However, the specific type of cancer could be treated and the outlook for patients was good.

In contrast, more commonly-occurring breast cancers were found no more frequently in women who had taken HRT.

'Studies rule out significant risk'

Publishing their findings in the Journal of the American Medical Association, they said: "We observed a positive, dose-response relationship between duration of postmenopausal hormone use and incidence of breast cancer with a favourable prognosis.

"This relationship appeared to be stronger for current users compared with past users."

An accompanying editorial said: "At this time the absence of convincing evidence of an association between ERT and breast cancer risk should be reassuring."

It said that there had been enough well-constructed, large-scale studies to have detected a significant risk if one existed.

'Excellent treatment'

Professor Gordon McVie, of the Cancer Research Campaign, agreed.

He told BBC News Online: "82 cancers out of 37,100 women? That's about the same risk as passive smoking.

"It doesn't change our general advice on HRT - that it's an outstanding drug treatment for postmenopausal symptoms."

He said doctors should counsel women on the risks before they started on the therapy and such women should remember to go for regular breast scans.