For the prevention of osteoporotic fracture, adequate screening and treatment are important. However, there are few published data on diagnosis and treatment rates of osteoporosis in Asia. We used data from the fourth Korea National Health and Nutrition Examination Survey 2008-2009 to estimate the nationwide prevalence, physician diagnosis rate, and treatment rate of osteoporosis in adults aged 50 years and older. The bone mineral density (BMD) measurements of central skeletal sites (lumbar spine, femoral neck, and total hip) were obtained using dual-energy X-ray absorptiometry (DXA) (Discovery-W; Hologic Inc., Waltham, MA, USA). Diagnosis of osteopenia or osteoporosis was defined by the World Health Organization (WHO) T-score criteria. The prevalence of osteoporosis in adults aged 50 years or older was 35.5% in women and 7.5% in men. The prevalence of osteoporosis in Korea was similar to other East Asian countries but higher than that in Caucasians. Lumbar spine bone density T-scores tended to be lower than those of the femoral neck or hip. The estimated diagnosis rate was 26.2% (women 29.9%, men 5.8%) and the treatment rate was 12.8% (women 14.4%, men 4.0%). The physician diagnosis rate was significantly higher in females aged 66 to 68 years who were the beneficiaries of the national screening program than that in females of other ages (43.6% versus 28.1%, p < 0.05). The national screening program for osteoporosis may have contributed to an increased diagnosis rate in older Korean women. However, it was evident that treatment following a diagnosis of osteoporosis was still inadequate.