Data supporting a role for RANKL signaling in mammographic density and breast cancer development has begun to emerge, but clinical trial data providing definitive evidence that would allow the adoption of RANKL inhibition in primary breast cancer prevention are not yet available. The hypothesis is that RANKL inhibition with denosumab will decrease mammographic density in high-risk premenopausal women with dense breasts. To address this, the investigators have developed this clinical trial to quantify the impact of RANKL inhibition on mammographic density in high-risk premenopausal women with dense breasts and to determine the effect of RANKL inhibition on markers of proliferation and biomarkers of breast cancer risk. Successful demonstration that RANKL inhibition reduces mammographic density could open up additional approaches to primary breast cancer prevention in high-risk premenopausal women, who do not have dominant genetic predisposition.