79 patients with osteogenesis imperfecta (OI) were randomized to receive either teriparatide (recombinant human parathyroid hormone) or a placebo administered subcutaneously daily for 18 months. The purpose of this proof-of-concept study was to determine whether teriparatide increased bone formation and positively affected skeletal mass and strength versus a placebo. The results at 18 months indicated that teriparatide significantly increased areal bone mineral density at the lumbar spine and total hip, as well as other bone formation markers. Post-hoc testing revealed significant differences between groups in type 1 OI patients, but not in type III/IV OI patients. Fracture risk was found to be similar, but the study was not adequately powered to detect a difference in fracture risk.