Serial blood gas measurement must be made for at least three months following an acute exacerbation of chronic obstructive pulmonary disease, to secure stable arterial oxygen tension (PaO2) values. The borderline indications of oxygen therapy concern the patients who have PaO2 over 8 kPa (60 mmHg), but have nocturnal hypoxaemia and/or pulmonary hypertension. The indications of specific drugs such as almitrine are still subject to discussion. The presence of obstructive sleep apnoeas together with ventilatory troubles related to chronic obstructive lung disease (overlap syndrome), is associated with many diagnostic and therapeutic problems.