Patients presenting with chronic obstructive airways disease and hypoxic cor pulmonale were assessed during a period of clinical stability. Seventy two patients (53 male) with a mean age of 60 years were selected for long term oxygen therapy. Mean FEV1 was 0.78 l and forced vital capacity 1.9 l. The mean arterial oxygen tension (PaO2) was 6.1 kPa (46 mm Hg) and the mean arterial carbon dioxide tension (PCO2) 6.9 kPa (52 mm Hg). All patients had a PaO2 of less than 8.0 kPa (60 mm Hg) and 57 patients had a PCO2 of more than 6.0 kPa (45 mm Hg). Pulmonary haemodynamics were measured in 45 patients yielding the following mean values: pulmonary artery pressure 28.3 mm Hg; cardiac output 5.9 l min-1; total pulmonary vascular resistance 59.2 kPa l-1 S. Oxygen delivery systems, including 23 oxygen concentrators, were installed in the patients' homes. Flow rates were adjusted to raise PaO2 to more than 8.0 kPa (60 mm Hg) for at least 15 hours each day and close supervision was maintained. Overall five year survival was 62%, which is better than previously reported for this type of patient; but the 10 year survival was only 26% owing to an observed acceleration in death rate at about this time. Progressive disturbances of the pulmonary circulation were arrested. Mortality was associated with the severity of airflow obstruction, reflecting a continuing pathological process affecting the airways.