In COPD patients with desaturation during exercise, several studies have shown an acute beneficial effect of supplemental oxygen. Therefore, both the British Thoracic Society and the American Thoracic Society guidelines recommend supplemental oxygen to patients who desaturate >4% and to below 88-90%. However, long-term studies have not been able to support this intervention. In this study of COPD patients, who desaturate and participate in a 7-week pulmonary rehabilitation programme, we examine the effect of supplemental oxygen. Using a randomised design, patients trained either with room air (control) or with 2 litres oxygen per minute from a portable oxygen concentrator. All patients were asked to exercise every day for 30 minutes.