C orticosteroid-induced osteoporosis is the most common secondary cause of osteoporosis. We conducted a 12-mo, randomized clinical trial of human parathyroid hormone 1-34 (hPTH 1-34) in postmenopausal women (mean age was 63 yr) with osteoporosis who were taking corticosteroids and hormone replacement therapy. Response to the treatment was assessed with bone mineral density (BMD) measurements of the lumbar spine by quantitative computed tomography (QCT); BMD measurements of the lumbar spine, hip, and forearm by dual-energy x-ray absorptiometry (DXA); and biochemical markers of bone turnover. The mean (+/-SE) changes in BMD of the lumbar spine by QCT and DXA in the PTH group were 35+/-5.5% and 11+/-1.4%, respectively, compared with a relatively small change of 1.7+/-1.8% and 0+/-0.9% in the estrogen-only group. The differences in mean percentage between the groups at 1 yr were 33.5% for the lumbar spine by QCT (P < 0.001) and 9.8% for the lumbar spine by DXA (P < 0.001). The changes in the hip and forearm were not significantly different between or within the groups. During the first 3 mo of PTH treatment, markers of bone formation increased to nearly 150%, whereas markers of bone resorption increased only 100%, suggesting an early uncoupling of bone turnover in favor of formation. These results suggest that parathyroid hormone dramatically increases bone mass in the central skeleton of postmenopausal women with corticosteroid- induced osteoporosis who are taking hormone replacement.