We evaluated the number of osteoporosis patients under treatment and secular trends in 2005-2008 in South Korea. We investigated nationwide data regarding the number of osteoporosis patients under treatment in South Korea using data from the Health Insurance Review and Assesment Service (HIRA), which includes nationwide information [corrected]. Reimbursement records from the HIRA database between 1 January 2004 and 31 December 2008 were investigated. Patients aged ≥30 years old with osteoporosis were identified based on a study-defined algorithm using prescription data and diagnostic codes. During the study periods, the number of patients receiving medical treatment related to osteoporosis increased from 1,034,399 to 1,392,189 for women and from 120,496 to 171,902 for men. The calculated proportion of osteoporosis patients under treatment in the general population over 50 years of age was 6.1% for men and 33.3% for women, and in the general population over 30 years of age was 2.7% for men and 16.6% for woman. More than 40% of patients (59.1% for women; 41.2% for men) were treated with medication indicated only for osteoporosis. About 4-7% of osteoporosis patients had a past medical history suggesting a secondary cause of osteoporosis. More than 80% of all osteoporosis patients were women older than 50 years, reflecting the pronounced burden of osteoporosis among postmenopausal women. This study demonstrated a substantial increasing trend in medical claims related to osteoporosis in 2005-2008 among adults in Korea and a pronounced burden of osteoporosis among postmenopausal women.