Domiciliary long-term oxygen therapy (LTOT) is a routine modality of treatment in advanced chronic obstructive pulmonary disease (COPD). More than 1 million patients worldwide receive LTOT. The patients who are eligible for LTOT are those who, in the steady state, present with severe hypoxaemia (arterial oxygen tension (Pa,O2 < or = 7.3 kPa (55 mmHg). Patients with a Pa,O2 of 7.4-7.8 kPa (56-59 mmHg) are also eligible if such hypoxaemia is accompanied by an elevated haematocrit and clinical signs of cor pulmonale. LTOT was found to prolong life expectancy, improve sleep, cognitive functions and emotional status and prevent the progression of hypoxic pulmonary hypertension. It seems that such effects apply to patients with severe hypoxaemia. Recent studies have demonstrated that, in patients with moderate hypoxaemia, Pa,O2 > 7.3 kPa (55 mmHg), LTOT does not prolong life. The effects of LTOT on quality of life in that group of patients remain to be elucidated. Some chronic obstructive pulmonary disease patients with a satisfactory arterial oxygen tension at rest and awake desaturate during sleep and receive nocturnal oxygen supplementation. The patients who qualify for oxygen treatment during sleep are those in whom arterial oxygen saturation during sleep falls below 90% for > or = 2 h. The long-term physiological effects of oxygen administered only during sleep are controversial. Some data suggest that oxygen supplementation in patients desaturating during sleep prevents the progression of hypoxic pulmonary hypertension and prolongs life. Other studies do not confirm those findings.