Women on oestrogen-only hormone replacement therapy face no increased breast cancer risk, research suggests.
Hormone replacement therapies come in a number of forms
This type of HRT is usually reserved for women who have had hysterectomies, as it increases womb cancer risk
The US study, published in the Journal of the American Medical Association looked at more than 10,000 women in this situation.
It follows previous studies which found the combined oestrogen progestogen HRT increased breast cancer risk.
Earlier research had suggested that the oestrogen-only HRT may also increase the risk of breast cancer.
In this latest study, from the long-term Women's Health Initiative trial by Stamford University researchers, women, aged between 50 and 79, were either given a type of oestrogen-only HRT called conjugated equine oestrogen (CEE) or a dummy version of the drug.
They were then monitored during a seven-year follow-up period through mammography screenings and clinical breast examinations.
A total of 104 breast cancer cases were reported among the women given oestrogen- HRT and 133 among the group given the dummy drug.
However, the study did find an increase in the number of mammograms requiring follow-up after the first year in the HRT group.
Experts said treatment with oestrogen-only HRT should only be started after all the potential risks and benefits for each individual had been weighed up.
Research leader Dr Marcia Stefanick said taking oestrogen-HRT alone for seven years "does not increase breast cancer incidence in postmenopausal women with hysterectomy, and may decrease the risk of early stage disease and ductal carcinoma [the most common form of breast cancer]."
She said the result was in clear contrast to the Women's Health Institute trial of combined progestogen and oestrogen therapy in women with a uterus.
This showed a significant increase in breast cancer rates over a 5.6 year period.
Experts said women should talk to their doctors to decide if either form of HRT was right for them.
Professor John Toy, medical director of Cancer Research UK, said the report strengthened the already well established message that HRT was a medicine which should be taken only after women and their doctors have carefully considered its potential medical benefits and risks.
He added: "Although this particular study shows no increase in breast cancers from oestrogen-only HRT - in contrast to other large studies which do show an increase - there are still considerable side-effects associated with it.
"There was a 50% increase in abnormal mammograms and a 33% increase in the number of breast biopsies required.
"These are both very worrying experiences for women to endure. And when breast cancers did occur they tended to be larger tumours spreading to lymph nodes."