Women taking hormone replacement therapy have an increased risk for developing ovarian cancer.
The study of nearly one million post menopausal women in the UK found those who were taking hormones had a 20% increased risk for ovarian cancer.
The risk continued to increase the longer a woman took them.
Experts calculate there were one thousand additional deaths from ovarian cancer due to hormone replacement therapy, but other factors could have played role.
Women who stopped therapy were no longer at risk.