Increasing numbers of postmenopausal women are undergoing hormonal replacement therapy. In this pictorial essay, we present the spectrum of mammographic changes seen in these women. These changes include symmetric and asymmetric increase in breast density, increase in size of fibroadenomas, and development or increase in size of cysts. Our examples illustrate that differentiation of hormonal therapy changes from neoplasm can occasionally be problematic when a focal density or mass is seen on mammograms. Furthermore, reexamination of published reports and our cases suggests that treatment with estrogen alone promotes enlargement of cysts and fibroadenomas, whereas treatment with a combination of estrogen and progesterone is more likely to be associated with diffuse increase in density.