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Objective: To compare the efficacy and safety of denosumab to ibandronate in postmenopausal women with low bone mineral density (BMD) previously treated with a bisphosphonate., Methods: In a randomized, open-label study, postmenopausal women received 60 mg denosumab subcutaneously every 6 months (n=417) or 150 mg ibandronate orally every month (n=416) for 12 months. End points included percentage change from baseline in total hip, femoral neck, and lumbar spine BMD at month 12 and percentage change from baseline in serum C-telopeptide at months 1 and 6 in a substudy., Results: At month 12, significantly greater BMD gains from baseline were observed with denosumab compared with ibandronate at the total hip (2.3% compared with 1.1%), femoral neck (1.7% compared with 0.7%), and lumbar spine (4.1% compared with 2.0%; treatment difference P<.001 at all sites). At month 1, median change in serum C-telopeptide from baseline was -81.1% with denosumab and -35.0% with ibandronate (P<.001); the treatment difference remained significant at month 6 (P<.001). Adverse events occurred in 245 (59.6%) denosumab-treated women and 230 (56.1%) ibandronate-treated women (P=.635). The incidence of serious adverse events was 9.5% for denosumab-treated women and 5.4% for ibandronate-treated women (P=.046). No clustering of events in any organ system accounted for the preponderance of these reports. The incidence rates of serious adverse events involving infection and malignancy were similar between treatment groups., Conclusion: In postmenopausal women previously treated with a bisphosphonate and low BMD, denosumab treatment resulted in greater BMD increases than ibandronate at all measured sites. No new safety risks with denosumab treatment were identified.