A study was undertaken to determine the usefulness of ubidecarenone in pulmonary rehabilitation in exercise training programs in the management of chronic obstructive pulmonary disease (COPD). The subjects were 20 patients with COPD who had been participating in an exercise training program for at least four weeks. The patients were randomly assigned either to receive 50 mg of oral ubidecarenone daily or to enter a control group during the program. Oxygen consumption, expired volume, and heart rate were measured during exercise tests before and after training. Maximum oxygen consumption increased 13% in the ubidecarenone-treated patients and 7% in the controls, and maximum expired volume increased 10% in each group. The increases were significant in the ubidecarenone group but not in the controls. Heart rate increased 2% in both groups. It is concluded that ubidecarenone deserves further evaluation in exercise training programs for patients with COPD.