The Women's Health Initiative agrees with some but not all case-control and cohort studies that current use of postmenopausal estrogen-progestin therapy is associated with a small increase in the risk of breast cancer. It is not known whether this is because of new tumor growth or an effect of hormonal therapy on preexisting tumors. Many studies indicate that women who develop breast cancer while using postmenopausal hormone therapy have a reduced risk of dying from breast cancer; this is consistent with an effect on preexisting tumors so that tumors appear at a less virulent and aggressive stage.