The objective of the study is to evaluate quantitative changes in mammographic breast density from baseline to 24 months in postmenopausal women receiving daily doses of either BZA 20 mg/CE 0.45 mg, BZA 20 mg/CE 0.625 mg, Raloxifene or placebo. The primary endpoint in this study is the change in mammographic breast density between baseline and month 24 for each group.