Long-term use of oestrogen-only hormone replacement therapy (HRT) does increase the risk of breast cancer, a major study suggests.
Hormone replacement therapies come in a number of forms
The US study comes just weeks after research appeared to rule out any connection in the short to medium term.
Oestrogen-only HRT is usually reserved for women who have had hysterectomies, as it increases womb cancer risk
The latest study, published in Archives of Internal Medicine, examined data on nearly 29,000 women.
Research has suggested that HRT using a combination of the hormones oestrogen and progestogen may increase the risk of breast cancer.
And some studies have also suggested a similar risk is associated with the oestrogen-only form.
However, a study of more than 10,000 women by Stamford University published last month found no evidence of any increased risk in women who used the therapy for up to seven years.
The latest study, by Brigham and Women's Hospital and Dana Farber Cancer Institute, Boston, followed a group of female nurses who took part in a long-term study which began in 1976.
Throughout the study period, 934 women developed invasive breast cancers. Of these 226 had never used hormones, and 708 had used oestrogen therapy.
The longer a woman used oestrogen, the higher her risk of breast cancer appeared to be.
Those who had been taking oestrogen for fewer than 10 years did not appear to have a higher risk than those who had never taken hormones.
But those who had been taking the hormone for more than 20 years had a significantly increased risk.
Henry Scowcroft, of the charity Cancer Research UK, said it was likely that that oestrogen-only HRT slightly increased a woman's chances of breast cancer when used over a long period.
He said a major study by the charity published in 2003 estimated that over 10 years, oestrogen-only HRT would lead to five additional breast cancers for every 1,000 women taking the treatment.
This compared with an estimated extra 19 breast cancers per 1,000 women taking combined HRT.
But he added: "HRT still remains an effective short-term treatment for relief of menopausal symptoms."
Emma Taggart, of the charity Breakthrough Breast Cancer, said: "Women should be aware that not all the causes of breast cancer are known and it's likely to be a combination of several factors which interact to increase risk.
"Whether to start or continue taking HRT needs to be an informed choice made with the help of a GP and should be reviewed every year."
Liz Carroll, of the charity Breast Cancer Care, agreed that women should discuss treatment options with their doctor.
But she added: "All women should be breast aware, by knowing how their breasts normally look and feel and reporting any changes immediately.
"The risk of breast cancer increases with age whether you take HRT or not."