OBJECTIVE: To assess the efficacy and safety of 18 months of subcutaneous abaloparatide (ABL-SC) or placebo (PBO) followed by 6 months of alendronate (ALN) (preplanned interim analysis). PATIENTS AND METHODS: ACTIVExtend, an extension of ACTIVE, enrolled patients who completed 18 months of ABL-SC or PBO in ACTIVE to receive up to 24 additional months of open-label ALN; there was 1 month between the studies to re-consent patients. RESULTS: Of 1243 eligible ACTIVE patients, 1139 (92%) were enrolled in ACTIVExtend beginning November 20, 2012. These results are from a prespecified 6-month interim analysis (cutoff date, June 2, 2015); the study is ongoing. Findings indicated percentages of patients with new morphometric vertebral fractures: PBO/ALN, 4.4% vs ABL-SC/ALN, 0.55%; relative risk reduction, 87% (relative risk, 0.13; 95% CI, 0.04-0.41; P<.001). Kaplan-Meier estimated rates of nonvertebral fractures were PBO/ALN, 5.6% vs ABL-SC/ALN, 2.7%; risk reduction, 52% (hazard ratio [HR], 0.48; 95% CI, 0.26-0.89; log-rank P=.02). There was also a 58% risk reduction of major osteoporotic fractures (HR, 0.42; 95% CI, 0.21-0.85; log-rank P=.01) and a 45% risk reduction of clinical fractures (HR, 0.55; 95% CI, 0.33-0.92; log-rank P=.02) in the ABL-SC/ALN group vs the PBO/ALN group. At 25 months, bone mineral density percentage change from ACTIVE baseline for ABL-SC/ALN vs PBO/ALN was as follows: lumbar spine, 12.8%; total hip, 5.5%; femoral neck, 4.5% vs 3.5%, 1.4%, 0.5%, respectively (group differences at all sites P<.001). CONCLUSION: Use of ABL-SC for 18 months followed by ALN for 6 months improved bone mineral density and reduced fracture risk throughout the skeleton and may be an effective treatment option for postmenopausal women with osteoporosis. TRIAL REGISTRATION: clinicaltrials.gov Identifier: NCT01657162.