643 postmenopausal women with osteoporosis who had been administered oral bisphosphonates for 2 or more years prior to enrollment were randomized to be administered either denosumab or zoledronic acid. The purpose of this study was to compare the change in bone mineral density (BMD) and bone turnover markers after 1 year. Results demonstrated a significantly greater percent increase in BMD - including lumbar spine, total hip, femoral neck, and distal radius - in the denosumab group compared to the zoledronic acid group. The percent reduction in bone turnover markers of serum C-telopeptide of type I collagen and procollagen type 1N-terminal propeptide was also significantly greater in the denosumab group compared to the zoledronic acid group.