Parathyroid hormone (PTH) is the major hormonal regulator of calcium homeostasis. PTH is a potent stimulator of bone formation and can restore bone to an osteopenic skeleton, when administered intermittently. Osteoblasts are the primary target cells for the anabolic effects of PTH in bone tissue. Anabolic effects of PTH on bone have been demonstrated in animals and humans, by numerous measurement techniques including bone mineral density and bone histomorphometry. Clinically, the most important aspect of treatment for osteoporosis is prevention of fractures. Microstructural alterations, such as loss of trabecular connectivity, have been implicated in increased propensity for fracture. Recent two-dimensional (2D) and three-dimensional (3D) assessments of cancellous bone structure have shown that PTH can re-establish lost trabecular connectivity in animals and humans. These results provide new insight into the positive clinical effects of PTH in osteoporosis. In recent randomized controlled clinical trials of intermittent PTH treatment, PTH decreased incidence of vertebral and non-vertebral fractures in postmenopausal women. Thus, PTH shows strong potential as therapy for osteoporosis. However, 2D and 3D structural analysis of advanced osteopenia in animals has shown that there is a critical limit of trabecular connectivity and bone strength below which PTH cannot completely reverse the condition. Given that PTH treatment fails to completely restore trabecular connectivity and bone strength in animals with advanced osteopenia, early treatment of osteoporosis appears important and efficacious for preventing fractures caused by decreased bone strength resulting from decreased trabecular connectivity.