In order to assess ventilatory control in patients with chronic airflow obstruction (CAO), the present study was carried out in nine patients with chronic obstructive pulmonary disease (COPD), eight asthmatics and nine normal subjects. We analysed the components of the respiratory control system at three levels: neural, assessed by diaphragmatic electromyography (EMGd), muscular, assessed by mouth occlusion pressure (P0.1), and ventilatory, assessed by mean inspiratory flow (VT/TI). EMGd was recorded by surface electrodes. During a CO2 rebreathing test, patients showed a normal or greater EMGd response slope (EMGdS), while for a given degree of EMGdS, P0.1 response slope (P0.1S) was found to be significantly reduced; in contrast, for a given degree of P0.1S, VT/TI response slope (VT/TIS) was found to be significantly reduced in COPD patients only. These data show that, compared to normal subjects, patients with CAO have a normal or increased neural component of the respiratory activity (EMGdS) and a relatively lower neuromuscular coupling (P0.1S/EMGdS). Probably due to different parenchymal and airway involvement, musculoventilatory transfer (VT/TIS/P0.1S) was found to be reduced in COPD patients but not in asthmatics. A complementary study, showing a good agreement between surface and oesophageal EMGd seems to confirm that surface EMGd is a useful and promising tool for clinical investigation.