• Compare the PTH-associated change from baseline at Month 3 of procollagen peptide P1NP, in subjects previously treated with Risedronate or Alendronate [ Time Frame: at Month 3 ] [ Designated as safety issue: No ]
  

• Compare changes from baseline for subjects previously treated with Risedronate or Alendronate for: P1NP & other biomarkers including bone-specific alkaline phosphatase, osteocalcin, serum CTX, urine NTX [ Time Frame: at 0.5, 1, 2, 3, 4, 5, 6, & 12 months of treatment ] [ Designated as safety issue: No ]
  
• Compare changes from baseline for subjects previously treated with Risedronate or Alendronate for: Lumbar spine & hip BMD measured by DXA [ Time Frame: after 6 & 12 months of treatment ] [ Designated as safety issue: No ]
  
• Compare changes from baseline for subjects previously treated with Risedronate or Alendronate for: Bone quality parameters captured by central quantitative computed tomography (QCT) [ Time Frame: after 12 months of treatment ] [ Designated as safety issue: No ]
  

All subjects will be treated with teriparatide (human, recombinant PTH[1-34])(human, recombinant PTH[1-34]), 20 microgram subcutaneously daily for 12 months. Subjects will consist of 290 postmenopausal women previously treated with either risedronate or alendronate for at least 24 months. An approximately equal number of subjects will have been previously treated with risedronate and alendronate, and the subjects will be balanced with regard to duration of previous treatment.

ELIGIBILITY CRITERIA include:

• Post-menopausal women who have used risedronate or alendronate continuously for at least 24 mos prior to enrollment
• Dosing regimens allowable are continuous (ie, uninterrupted) daily or weekly formulations of risedronate (5 mg once daily [OD] or 35 or 30 mg once a week [OAW]) or alendronate (10 mg OD or 70 mg OAW), for a minimum of 24 months prior to enrollment into study
• Lumbar spine or total hip BMD T-score 1ess than or equal to -2.0 and >/= 1 prevalent osteoporotic fracture, or lumbar spine or total hip BMD T-score less than or equal to -2.5 with or without and >/= 1 prevalent osteoporotic fracture. The qualifying values must be documented prior to enrollment
• Vitamin D (25-hydroxyvitamin D) between 16 ng/ml and 80 ng/ml
• Urine NTX <50 nmol/mmol creatinine (to assure treatment compliance and bone turnover is in the pre-menopausal range)

EXCLUSION CRITERIA include:

• Impaired renal function, demonstrated by creatinine clearance < 30 ml/min
• Any condition or disease that may interfere with the evaluation of at least 2 lumbar vertebrae (not necessarily contiguous), determined in a screening radiograph by a radiologist at the central facility (eg, confluent aortic calcifications, severe osteoarthritis, spinal fusion, lumbar spine fractures)
• Depot injection vitamin D >10,000 IU in the past 9 months prior to starting the investigational product
• Treatment with antiresorptive agents other than risedronate, alendronate, and hormone replacement therapy within the last 36 months before study entry (ie, ibandronate, pamidronate, etidronate, raloxifene, clodronate, or zoledronate)
• Use of combination alendronate and risedronate, either simultaneously or sequentially, within 60 months prior to enrollment, or use of any anti-resorptive agent in combination with risedronate or alendronate