The value of a programme of graded exercise training was assessed in ten subjects with chronic obstructive airways disease. Assessments were made at two-weekly intervals during three twelve-week periods: before, during and after a period of training. During the training the subjects were able to walk significantly further in a 12 minute walking test. There was also a small but significant increase in the forced expiratory volume in one second and the difference between the values of total lung capacity estimated by body plethysmography and those estimated by helium dilution was significantly smaller. The transfer factor for carbon monoxide increased and was significantly greater in the period after stopping the training. The difference between alveolar and arterial oxygen tensions at rest was smaller during the training period. The changes in the results of the 12 minute walking test, transfer factor and the oxygen tension difference were maintained in the period after stopping the training. Despite the improvements in some of the indices of respiratory function there were no improvements in any of the indices of cardiorespiratory fitness which may be because the subjects were incapable of undertaking sufficiently intense training. It is concluded that training is of some benefit to patients with chronic obstructive airways disease but that the benefit may be greater in patients who are less incapacitated and who are able to undertake a more strenuous training programme.