Taking HRT can double a woman's risk of developing breast cancer, scientists have warned.
The researchers looked at a particular type of HRT
Women over 65 had 1.7 times the risk of developing the cancer if they had taken a combination of oestrogen and progestin for over five years, compared to if they had taken no HRT.
Their increased risk of developing some specific types of tumour was even higher.
In contrast, the study of 2,000 women showed those who took oestrogen-only HRT for 25 years or longer had no increased risk of developing breast cancer.
The US researchers were looking at a version of hormone replacement therapy which combines progestin and oestrogen.
Similar combinations are available in Europe, although they do not use the same type of progestin.
However, most women taking HRT in the UK use a combined version.
UK experts said there was an increased breast cancer risk for women taking the combined HRT, which had to be considered when they decided whether they should take the medication.
But they said that, per year, the increased risk was the same as having a late menopause.
A second US study of 16,000 postmenopausal women has shown breast cancers in women taking the combined HRT were more advanced at the time of diagnosis.
Half the women were given the HRT, the rest a dummy pill, or placebo.
In the HRT group, 199 had invasive breast cancer, compared to 150 in the other group.
Over 700 women taking HRT were found to have abnormal mammograms compared to 398 taking the dummy pill.
The researchers suggest the oestrogen/progestin combination stimulates tumour growth and hinders diagnosis.
In 2002, the study was stopped three years early because of concerns over the potential effects of taking the combination HRT.
In addition to breast cancer, early results from the study showed women were at an increased risk of heart disease and stroke.
Writing in the Journal of the American Medical Association, the researchers led by Rowan Chlebowski of Harbor-UCLA Research and Education Institute
in Torrance, California, said: "Combined oestrogen plus progestin use increases the risk of breast cancers, which are diagnosed at a more advanced stage compared with placebo use.
"In light of these findings, abnormal mammograms in women receiving menopausal hormone therapy deserve heightened scrutiny."
Mr Malcolm Whitehead, a UK expert in HRT, told BBC News Online he had some concerns about how the two studies had been carried out.
But he said: "For women on combined HRT treatment, there is effectively a 2.5% excess risk for each year of use. That's the same risk that you've got with a late menopause.
"If you take HRT, you have to accept the risk.
"But women should put it into perspective; one unit of alcohol a day increases a woman's risk of breast cancer by 6%."
He said not having children gave women an extra lifestyle risk of developing the cancer of between 70 and 100%.
Mr Whitehead said many of women whose mammograms had shown abnormalities in the US study had not had breast cancer.
But he added: "If patients are worried that taking HRT might reduce the chance of having their breast cancer being detected at their next screening, they could stop taking their HRT for two weeks before having their mammogram."