Long-term oxygen therapy improves the life expectancy of hypoxaemic patients with chronic obstructive pulmonary disease (COPD), provided the hypoxaemia is sufficiently pronounced under stable conditions (PaO2 less than 55 mmHg) and oxygen is given for more than 16 out of 24 hours. By extension, the same indications are applicable to hypoxaemia due to other causes (diffuse fibrosis, pneumoconiosis, cystic fibrosis, etc.). Long-term oxygen therapy improves the patients' quality of life and also has favourable effects on oxygen transport, neuropsychological status, polycythaemia and pulmonary hypertension. It is usually delivered by means of O2 extractors, to which may be added small flasks of O2 gas for walking and moving about. Liquid O2 is a good solution for subjects who are motivated and are obliged to do a great deal of walking. The O2 flow rate administered must be such that it rises the PaO2 level above 60 mmHg. Oxygen therapy is only a symptomatic treatment and cannot replace other types of therapy, such as bronchodilators, physiotherapy, etc. It gives satisfactory results but it has not transformed the prognosis of severe hypoxaemic COPD.