Estrogen alone, progestogen alone, and combined estrogen and progestogen therapy are all associated with increased risks of breast cancer when used in menopausal hormone therapy for peri- and postmenopausal women relative to non-use. These risks are higher for combined estrogen and progestogen therapy than with estrogen alone or progestogen alone. In addition to breast cancer risk, estrogen alone and estrogen plus progestogen therapy are associated with higher breast cancer mortality. With 20 years of use, breast cancer incidence is about 1.5-fold higher with estrogen alone and about 2.5-fold higher with estrogen plus progestogen therapy relative to non-use. The increase in breast cancer risk with estrogen and progestogen therapy was shown to be causal with conjugated estrogens plus medroxyprogesterone acetate in the Women's Health Initiative randomized controlled trials.