Individual epidemiologic studies and a meta-analysis of those studies suggest that the relative risk of invasive lobular and compared to invasive ductal cancer is particularly affected by combined hormone replacement therapy (HRT) in postmenopausal women, with an approximate doubling of risk of invasive lobular breast cancer in current users. However, this was not seen in the Women's Health Initiative trial comparing combined HRT and placebo, although this trial had only modest statistical power to demonstrate the effect. In the absence of some obvious source of bias (screening mammography rates were well balanced between HRT users and non-users in most studies), this evidence supports the hypothesis that combined HRT increases the risk of lobular breast cancer.