 long-term oxygen therapy prolongs life in patients with severe COPD and severe resting hypoxemia. The objective of this study is to measure the survival in relation to the level of initial resting hypoxemia, serious co-morbidities, and nutritional status in COPD patients treated by long-term oxygen therapy. After enrollment at the beginning of oxygen therapy, following data will be collected during 3 years. Lung function, blood gas analysis, exercise capacity measured by the 6-Min Walk Distance test 6-MWD, dyspnea scale, BODE index, quality of life, indirect and direct costs will be analyzed every year. 