We used transcranial magnetic stimulation to evaluate cortical excitability in acute exacerbations of chronic obstructive pulmonary disease (COPD). Patients affected by COPD were studied during acute exacerbation that required hospital admission and 3-4 months after oxygen therapy. Their data were compared with those of age-matched healthy controls. Intracortical inhibition and cortical silent period duration were significantly reduced in patients during acute exacerbation of COPD. These findings could reflect impairment of GABAergic cortical circuits during hypoxia.