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Tsai-Yu Wang,1 Yu-Lun Lo,1,2 Pai-Chien Chou,1 Fu-Tsai Chung,1 Shu-Min Lin,1 Ting-Yu Lin,1 Horng-Chyuan Lin,1 Chun-Hua Wang,1 Chih-Teng Yu,1 Han-Pin Kuo1 1Department of Thoracic Medicine, School of Medicine, Chang Gung Memorial Hospital and Chang Gung University, Taipei, Taiwan; 2Healthcare Center, Chang Gung Memorial Hospital, Taipei, Taiwan Background: Osteoporosis is an important issue for patients with chronic obstructive pulmonary disease (COPD). Worse systemic inflammation and reduced exercise capacity have been reported in COPD patients with obstructive sleep apnea (OSA), implying that OSA may be an independent factor for osteoporosis in COPD patients. Methods: A total of 66 patients with bone mineral density (BMD) and polysomnography results from a previous COPD cohort (January 2008 to January 2013) were retrospectively enrolled. Clinical characteristics such as medication, pulmonary function, BMD, and results of polysomnography were analyzed. Results: The BMD in those with OSA was significantly lower than in those without OSA (-1.99±1.63 versus -1.27±1.14, P=0.045). In univariate analysis, body mass index, forced expiratory volume in 1 second, percentage of predicted value, incremental shuttle walk test, apnea–hypopnea index, and oxygen desaturation index (ODI) were significantly associated with BMD. After multivariate linear regression analysis, the ODI was still an independent factor for BMD. In addition, smaller total lung capacity is significantly associated with higher ODI and lower BMD, which implies that lower BMD might cause severer OSA via decreased total lung capacity. Conclusion: OSA may be an independent factor for BMD in patients with COPD, which implies a possible vicious cycle takes place in these patients. Keywords: chronic obstructive pulmonary disease, osteoporosis, total lung capacity&nbsp