Yasuhiro Yamauchi,1,2 Wakae Hasegawa,1 Hideo Yasunaga,3 Mitsuhiro Sunohara,1 Taisuke Jo,1,2 Kazutaka Takami,1 Hiroki Matsui,3 Kiyohide Fushimi,4 Takahide Nagase1 1Department of Respiratory Medicine, Graduate School of Medicine, The University of Tokyo, Tokyo, Japan; 2Division for Health Service Promotion, The University of Tokyo, Tokyo, Japan; 3Department of Clinical Epidemiology and Health Economics, School of Public Health, The University of Tokyo, Tokyo, Japan; 4Department of Health Policy and Informatics, Tokyo Medical and Dental University, Graduate School of Medicine, Tokyo, Japan Background and objective: The prevalence and mortality of chronic obstructive pulmonary disease (COPD) in elderly patients are increasing worldwide. Low body mass index (BMI) is a well-known prognostic factor for COPD. However, the obesity paradox in elderly patients with COPD has not been well elucidated. We investigated the association between BMI and in-hospital mortality in elderly COPD patients. Methods: Using the Diagnosis Procedure Combination database in Japan, we retrospectively collected data for elderly patients (>65 years) with COPD who were hospitalized between July 2010 and March 2013. We performed multivariable logistic regression analysis to compare all-cause in-hospital mortality between patients with BMI of <18.5 kg/m2 (underweight), 18.5–22.9 kg/m2 (low–normal weight), 23.0–24.9 kg/m2 (high–normal weight), 25.0–29.9 kg/m2 (overweight), and ≥30.0 kg/m2 (obesity) with adjustment for patient backgrounds. Results: In all, 263,940 eligible patients were identified. In-hospital mortality was 14.3%, 7.3%, 4.9%, 4.3%, and 4.4%, respectively, in underweight, low–normal weight, high–normal weight, overweight, and obese patients. Underweight patients had a significantly higher mortality than low–normal weight patients (odds ratio [OR]: 1.55, 95% confidence interval [CI]: 1.48–1.63), whereas lower mortality was associated with high–normal weight (OR: 0.76, CI: 0.70–0.82), overweight (OR: 0.73, CI: 0.66–0.80), and obesity (OR: 0.67, CI: 0.52–0.86). Higher mortality was significantly associated with older age, male sex, more severe dyspnea, lower level of consciousness, and lower activities of daily living. Conclusion: Overweight and obese patients had a lower mortality than low–normal weight patients, which supports the obesity paradox. Keywords: mortality, obesity paradox, COPD