Estrogen plus progestin increased the risk of breast cancer in postmenopausal women taking hormone therapy, U.S. researchers found. A review published in the Journal of The National Cancer Institute found in the past decade, results from large prospective cohort studies and the Women\'s Health Initiative randomized placebo-controlled hormone therapy trials have substantially changed thoughts about how estrogen alone and estrogen plus progestin influence the risk of breast cancer. Although hormone therapy is currently used by millions of women for menopausal symptoms, there is still concern about hormone therapy–induced breast cancer risk. In addition, the effects of estrogen plus progestin versus estrogen alone on breast cancer are not completely understood. Dr. Rowan T. Chlebowski of the Los Angeles Biomedical Research Institute at Harbor-University of California, Los Angeles, Medical Center and Garnet Anderson of the Fred Hutchinson Cancer Research Center analyzed the data from two randomized, placebo-controlled full scale clinical trials conducted in the Women\'s Health Initiative. One trial evaluated estrogen plus progestin in postmenopausal women with an intact uterus, and the other evaluated estrogen alone in postmenopausal women with prior hysterectomy. Estrogen plus progestin statistically significantly increased the risk of breast cancer, while, in contrast, estrogen alone use in postmenopausal women with a previous hysterectomy, statistically significantly decreased the risk of breast cancer, the researchers said. \"An imbalance in the use of mammography with greater screening for hormone users could explain some of the increase in breast cancer incidence with estrogen alone seen in cohort studies because screened populations have more cancers detected than unscreened populations,\" Chlebowksi said in a statement.