OBJECTIVE : Sarcopenia gradually progress with age; hence, it is necessary to define sarcopenia to predict postoperative outcomes in elderly patients with lung cancer. The purpose of this study is to propose a definition of sarcopenia in elderly patients with lung cancer, and to demonstrate the post operative outcomes. METHODS : Using computed tomography scans, the cross-sectional area (cm2) of the psoas muscle at the third lumbar vertebral level was measured. The psoas area was normalized for height. The psoas muscle mass index was calculated as total psoas muscle area at L3 level/height2 (cm2/m2). RESULTS : A total of 173 patients aged > 75 years of age received lobectomy for non-small cell lung cancer in our hospital. We defined sarcopenia as the psoas muscle mass index under 3.70 cm2/m2 in males and 2.50 cm2/m2 in females, based on the morbidity rate. The postoperative complication rate was significantly higher in patients with sarcopenia (62.5%) than in those without sarcopenia (22.7%). The 5-year survival rate was 26.5% in patients with sarcopenia, and 66.3% in patients without sarcopenia. CONCLUSIONS : In elderly lung cancer patients, sarcopenia was observed to be a high risk for morbidity and predicted poor prognosis.