Ambulatory oxygen therapy in patients with COPD and exercise hypoxaemia may improve exercise capacity and breathlessness. The improvement in exercise tolerance is related to the degree of exercise hypoxaemia. As effects from ambulatory oxygen therapy are prone to placebo effects, careful patient selection is required, with formal assessment. For effective ambulatory oxygen delivery, oxygen should be provided either by small liquid oxygen tanks or by small lightweight cylinders and conserving devices. Studies are required to evaluate the longer term effects of ambulatory oxygen on daily life.