HRT itself does not cause breast cancer. It can feed an already existing estrogen-driven cancer though. It's important to remember that in the WHI study, the 26% increase in breast cancer rates on HRT was actually a very, very small increase. It was sensationalized and the relative risk was reported not the absolute risk. In our daily lives, the risk of developing breast cancer between the ages of 50 and 60 is 2.33%. On HRT, according to that study, the risk increased from 2.33% to 2.94%. That's a tiny increase of 8 additional women per 10,000. And that was for women on Premarin – an oral estrogen from horse urine that contains a multitude of estrogens, along with synthetic progestin which some speculate might be the greater villain. Both are considered much less safe than transdermal estrogen and bioidentical progesterone so actual increased risk may even be smaller than was "concluded" by the study. For some people, any increase in risk is too much risk which I understand, particularly if there is previous breast cancer or a genetic risk. It's a really personal decision and one that should be explored with a menopausal specialist that has a thorough grasp of all the nuances.