The research, published in the New England Journal of Medicine, showed that the incidence of breast cancer dropped sharply when women stopped taking HRT. Some experts believe that the recent drop in the use of HRT has prevented as many as 1000 cases of breast cancer a year in the UK. However, other experts think that the fall in breast cancer rates may be caused by other factors.
The original 2002 study, the Women's Health Initiative, which suggested a link between the combined hormones used in HRT and breast cancer is highly controversial. It did, however, result in a lot of women stopping HRT in 2002, after which breast cancer rates fell in the US and in other countries.
The latest research from a group in California not only kept monitoring 15 000 women from the original study, who had all been urged to stop taking HRT in 2002, but looked at data for women not originally involved, who had been given no specific advice on giving up.
In the first group, the incidence of breast cancer was much higher in the hormone group in the five years leading up to 2002, then dropped very rapidly, with the number of diagnoses falling 28% in 12 months.
The women had roughly the same number of mammograms before and after 2002.
Many women in the second group also chose to stop taking the tablets, and this 50% decline in hormone use coincided with a 43% fall in breast cancer rates between 2002 and 2003.
Women in the second group who carried on taking HRT were at higher risk of cancer - with a woman who continued for five years doubling her breast cancer risk every year, according to the study.
Other experts however, are not convinced. Dr David Sturdee of the International Menopause Society who represents HRT specialists, says that breast cancer takes years to develop, so if the reduction in incidence was due to stopping HRT it wouldn't be seen yet.
Other experts are calling for more studies to see if the sudden drop in the incidence of breast cancer is seen elsewhere.
