CONTEXT: Bone strength and fracture resistance are determined by bone mineral density (BMD) and structural, mechanical, and geometric properties of bone. DESIGN, SETTING, AND OBJECTIVES: This randomized, double-blind, placebo-controlled outpatient study evaluated effects of once-monthly oral ibandronate on hip and lumbar spine BMD and calculated strength using quantitative computed tomography (QCT) with finite element analysis (FEA) and dual-energy x-ray absorptiometry (DXA) with hip structural analysis (HSA). PARTICIPANTS: Participants were women aged 55-80 yr with BMD T-scores -2.0 or less to -5.0 or greater (n = 93). INTERVENTION: Oral ibandronate 150 mg/month (n = 47) or placebo (n = 46) was administered for 12 months. OUTCOME MEASURES: The primary end point was total hip QCT BMD change from baseline; secondary end points included other QCT BMD sites, FEA, DXA, areal BMD, and HSA. All analyses were exploratory, with post hoc P values. Results: Ibandronate increased integral total hip QCT BMD and DXA areal BMD more than placebo at 12 months (treatment differences: 2.2%, P = 0.005; 2.0%, P = 0.003). FEA-derived hip strength to density ratio and femoral, peripheral, and trabecular strength increased with ibandronate vs. placebo (treatment differences: 4.1%, P < 0.001; 5.9%, P < 0.001; 2.5%, P = 0.011; 3.5%, P = 0.003, respectively). Ibandronate improved vertebral, peripheral, and trabecular strength and anteroposterior bending stiffness vs. placebo [7.1% (P < 0.001), 7.8% (P < 0.001), 5.6% (P = 0.023), and 6.3% (P < 0.001), respectively]. HSA-estimated femoral narrow neck cross-sectional area and moment of inertia and outer diameter increased with ibandronate vs. placebo (respectively 3.6%, P = 0.003; 4.0%, P = 0.052; 2.2%, P = 0.049). CONCLUSIONS: Once-monthly oral Ibandronate for 12 months improved hip and spine BMD measured by QCT and DXA and strength estimated by FEA of QCT scans.