OBJECTIVES: To evaluate the prevalence of osteoporosis in the Alzheimer's disease patients. METHODS: One hundred and twenty eight patients with Alzheimer's disease were included in this study. The patients are diagnosed with Alzheimer's disease using Korean-Mini Mental Status Examination (K-MMSE) and checked bone mineral density using single-energy (SE) quantitative computed tomography (QCT) in 2010. To define abnormality in terms of QCT of the spine, a bone mineral density (BMD) spine below 80 mg/cm3 is indicative of osteoporosis and BMD between 80 and 120 mg/cm3 is indicative osteopenia. RESULTS: The prevalence of osteoporosis in the control group is 29.73%. The prevalence of osteoporosis in the Alzheimer's disease patients is 96.87%. The prevalence of osteopenia in the Alzheimer's disease patients is 3.12%. The prevalence of severe osteoporosis fracture in the Alzheimer's disease patients is 32.03%. CONCLUSION: The prevalence of osteoporosis in Alzheimer's disease patients is higher than in the control group.