Many chronic obstructive pulmonary disease patients suffer from dyspnoea and reduced physical abilities. Some of the patients exhibit desaturation during physical exercise. Few studies have examined if oxygen therapy during physical exercise can relieve symptoms and improve outcomes of pulmonary rehabilitation. These studies show that oxygen therapy during exercise results in significant improvement in exercise capacity. Yet it is not possible to draw a clear conclusion regarding the effects of more prolonged (weeks) treatment. The studies are in general small and of poor methodological quality.