The effect of oxygen on breathlessness and exercise tolerance was examined in 'pink and puffing' patients with fixed airways obstruction. When breathing oxygen, patients were less breathless and walked further. This was true whether the cylinder was carried by the patient or by an assistant. It was not possible to identify those patients who would benefit most. The best method of assessing improvement was by comparing breathlessness on a standardised progressive exercise test on a treadmill. Four patients had a greater than 30% reduction in breathlessness on submaximum exercise when breathing oxygen. Breathing oxygen for five or fifteen minutes before exercise but not during exercise (predose) resulted in a similar improvement in exercise tolerance. For short periods of exercise predosing with oxygen provides a convenient alternative to continuous oxygen. For longer periods of exercise the benefits of portable oxygen in selected patients have been previously underestimated.