Fifty-four consecutive patients with COPD evaluated at the exercise physiology laboratory of Hopital Laval, Ste-Foy, Quebe and in whom serial arterial lactate measurements could be obtained during exercise were included in this study. In each case the diagnosis of COPD was based on previous or current smoking history and pulmonary function tests showing moderate to severe irreversible bronchial obstruction . Patients were clinically stable at the time of the study, and none suffered from cardiovascular, neurological or any other conditions that could impair capacity to perform an exercise test. Ten healthy, nonsmoking males, aged 58 to 70 years, were recruited by means of newspaper advertisement to serve as controls. None of these subjects was involved in a regular exercise program. The institutional ethics committee approved the research protocol, and informed consent was obtained from each patient.