Fourteen out of a group of 21 patients with cor pulmonale associated with obstructive airways disease received controlled oxygen therapy for 24 hours per day during acute exacerbations in hospital, but for only 12 hours per day at home in convalescence. Three patients derived benefit judged by a reduction in the expected number of acute exacerbations of cor pulmonale, but most patients continued to deteriorate. It was felt that 12 hours per day was an insufficient period for oxygen therapy and longer periods were necessary. Despite this poor response pulmonary artery pressure measured in convalescence was normal or only slightly elevated in many patients. The aetiology of cor pulmonale and in particular the importance of increased pulmonary vascular resistance in obstructive airways disease is discussed. The findings do not support the hypothesis that cor pulmonale is invariably due to pressure overload of the right ventricle.