New evidence that the risk of breast cancer is made worse by hormone replacement therapy (HRT) is contained in a study by researchers at Stanford University in California, USA.
It concludes that after five years of treatment with the hormones oestrogen and progestin, woman are doubly at risk of developing the disease.
And it supports the 2002 US Women's Health Initiative (WHI) trial, which established links between HRT and breast cancer, blood clots and strokes.
After the trial, although the use of HRT treatment and breast cancer rates in the USA both fell significantly, some experts still questioned the connection.
Stanford's Professor Marcia Stefanick, who chaired the WHI executive committee, said: "This is very strong evidence that oestrogen plus progestin causes breast cancer.
"You start women on hormones and within five years, their risk for breast cancer is clearly elevated. You stop the hormones and within one year, their risk is essentially back to normal. It's reasonably convincing cause-and-effect."
His team looked at data from 41,000 women enrolled in a separate observational study as well as the 15,000 women who took part in the original WHI trial.