Is there an association between hormone replacement therapy and breast cancer?

The fear of breast cancer has been suggested as a potential reason why only a relatively small percentage of postmenopausal women who would benefit from hormone replacement therapy (HRT) are current users. The equivocal results from a large number of epidemiologic studies make it difficult to evaluate whether an association does indeed exist between the use of HRT and the incidence of breast cancer. The inability to provide conclusive evidence for or against this relationship may be attributed to methodologic problems in these studies, including small sample sizes, lack of information on specific hormonal preparations (e.g., dose and type), failure to control for the type of menopause, and surveillance bias. In an attempt to generalize results from different studies in a systematic manner, several meta-analyses have been conducted of the effects of estrogen replacement therapy (ERT) or HRT on the risk of breast cancer. This article summarizes the data from these meta-analyses and incorporates data from studies published after these meta-analyses that have addressed this question. Data from ongoing studies that use a randomized, controlled, longitudinal design on large numbers of women are necessary before a possible association between the use of HRT and breast cancer can be ascertained.