Background: Body mass index (BMI) and fat free mass index (FFMI) is known to decrease in patients with chronic obstructive lung disease (COPD), and low BMI and FFMI are associated with worse lung function and poor prognosis. Data are limited on the relationship between BMI, FFMI and COPD in Asian countries. Method: We used the data in Korean National Health and Nutrition Survey (KNHANES) in year 2008 to 2010, 822 subjects with COPD and the same number control subjects were selected. Spirometry and Dual Energy X-Ray Absorptiometry (DXA) were done for the analysis. Results: Subjected with COPD had lower mean BMI and FFMI compared with control group (23.6 vs. 23.9 kg/m2 and 17.2 vs. 17.5 kg/m2, p=0.015 and p=0.013, respectively). The BMI and FFMI values decreased significantly in subjects with severe airway obstruction (p＜0.001). The proportion of subjects who are underweight (BMI ＜18.5 kg/m2) or with low FFMI (FFMI ＜5 percentile) was higher in GOLD stage 3-4 group (p＜0.001). The deficit of FFMI was more prominent in the lower ex-tremities, followed by the upper extremities and the trunk. Subjects who are underweight had lower FEV1 and FEV1/FVC values compared with each subgroups (p＜0.001). Conclusion: BMI and FFMI decreased in adults with severe COPD, and FFMI deficit was more prominent in the lower extremities. We suggest that the assessment of BMI and body composition is necessary in patients with severe airway ob-struction, and active rehabilitation has to be considered to preserve FFM for those patients. 보건복지부 보건의료연구개발사업 지원(A102065).