The purpose of the study is to determine the safety and effectiveness of 2 doses of the investigational drug, lasofoxifene, compared to placebo (an inactive substance) in reducing new spinal fractures in women with osteoporosis.

Inclusion Criteria:

Postmenopausal at least 5 years; Screening bone mineral density more than or equal to 2.5 S.D.s below the mean for young adults at the lumbar spine or femoral neck.

Exclusion Criteria:

Metabolic bone disease other than osteoporosis; Taking approved medications for osteoporosis; Have had a recent osteoporotic fracture (within 1 year) and/or have a bone mineral density more than 4.5 S.D.s below the mean for young adults at the lumbar spine or femoral neck.