Purpose : To evaluate mammographic and sonographic breast parenchymal changes and the risk of breast cancer inwomen on hormonal replacement therapy (HRT). Maerial & Methods : The study group consisted of 50 patients examinedwith serial mammogram and/or ultrasonogram during HRT. The control group consisted of 50 patients examined withserial mammogram for a routine health check. Mammographic parenchymal changes in both the studyn and controlgroups and so-nographic findings of 27/50 patients in study group were evaluated. Results : Follow-up mammogram ofthe control group showed no interval change or slight evolution of parenchyma with increasing age, but the studygroup showed increasing parenchymal densities. Most frequently encountered finding on sonogram in 11 women treatedby estrogen alone, was ductal dilatation ( 7cases; 64 % ), whereas in 16 women treated by estrogen and progesteronit was ductal epithelial hyperplasia ( 8 cases; 50 % ). Overall, four breast cancers developed; one infiltratingductal carcinoma and three ductal carcinoma in situ. Conclusion : HRT causes the changes of breast parenchyma onmammogram and sonogram of postmenopausal women, and increases the risk of developing breast cancer. Therefore, carful and regular examination should be followed in those on postmenopausal HRT.