552 Background: Adjuvant aromatase inhibitor (AI) therapy is well established in postmenopausal women with hormone receptor-positive breast cancer, but such therapy is associated with bone loss and increased fracture risk. Denosumab, a fully human monoclonal antibody against receptor of nuclear-κB ligand, was previously proven to protect against AI-induced bone loss. In Japan, however, the efficacy of denosumab in the treatment of AI-associated bone loss has not been proven in a prospective study. Methods: This non-randomized prospective study was conducted at four institutions in Japan. we prospectively evaluated the bone mineral density (BMD) of the lumbar spine and bilateral femoral neck in hormone-receptor positive clinical stageⅠ–ⅢA, postoperative postmenopausal breast cancer patients who were scheduled for treatment with AI as adjuvant endocrine therapy or during AI adjuvant therapy. They received supplemental calcium, vitamin D and subcutaneous denosumab 60mg (n=103) every six months. At enrollment, all patients were required to have evidence of low bone mass, excluding osteoporosis. The primary endpoint was percentage change in lumbar spine BMD from baseline to month 12. The secondary endpoint was percentage change in bilateral femoral neck BMD from baseline to month 12. This is the first trial where the right and left femoral neck BMD are measured separately. Results: We enrolled 103 patients between November, 2014 to October, 2016. At 12 months, lumber spine BMD increased by 4.7 %. The patients who were administered prior AI therapy (n=60) had a 4.8 % increase, and the patients without prior AI therapy (n=40) had a 4.6 % increase. At 12 months, the right and left femoral neck BMD increased by 2.9 % and 2.0 %, respectively. Hypocalcemia ≥ grade2, osteonecrosis of the jaw (ONJ) and non-traumatic clinical fracture were absent in this study. Conclusions: Twice-yearly treatment with denosumab was associated with consistently greater gains in BMD among Japanese women receiving adjuvant AI therapy, regardless of whether prior AI therapy was administered. Clinical trial information: UMIN000013863.