Background: Long-term oxygen therapy (LTOT) is the only component of the management of chronic obstructive pulmonary disease (COPD) that improves survival in patients with severe daytime hypoxemia (defined as an arterial oxygen pressure [paO2] measured in stable state <=55 mmHg or in the range 56-59 mmHg when clinical evidence of pulmonary hypertension or polycythemia are noted). In Canada, LTOT is usually provided by a stationary oxygen concentrator and is recommended to be used for at least 15-18 hours a day. Several studies have demonstrated a deterioration in arterial blood gas pressures and oxygen saturation during sleep in patients with COPD. Sleep-related oxygen desaturation often occurs in patients not qualifying for LTOT. The suggestion has been made that the natural progression of COPD to its end stages of chronic pulmonary hypertension, severe hypoxemia, right heart failure, and death is dependent upon the severity of desaturation occurring during sleep. This is an attractive hypothesis and is supported by the fact that hypoxemic episodes during sleep are accompanied by substantial increases in pulmonary arterial pressure and often by important cardiac arrhythmias. Supplemental nocturnal oxygen alleviates both the acute increases in pulmonary arterial pressure and the cardiac arrhythmias. It has been suggested that, over the long run, nocturnal oxygen therapy (N-O2) may halt the progression of long-standing cor pulmonale and prolong survival. Probably due to the fact that the recommendations of scientific societies regarding the indications for and use of N-O2 in COPD not qualifying for conventional LTOT are presently imprecise, a number of patients are currently treated with N-O2 although the beneficial effects of this therapy have not been confirmed.