PTH-related protein, or ''PTHrP'', is a hormone which was discovered in 1987. As its name implies, it is closely related to another hormone discovered in the 1920's named parathyroid hormone or ''PTH''. PTH has been shown to be effective in treating osteoporosis in both animals and humans. PTHrP has been shown to be effective in treating osteoporosis in laboratory animals, and there are strong scientific reasons to think that it may be effective in humans as well. However, no human trials with PTHrP in the treatment of osteoporosis have been performed. The studies in this trial are focussed on determining whether PTHrP can indeed increase bone mass in postmenopausal women with osteoporosis, when administered daily by subcutaneous injection for three months.