With the purpose of defining the pattern of abdominal respiratory muscle activity in patients with chronic obstructive pulmonary disease (COPD), we studied the electromyogram of the rectus abdominis (RA), the external oblique (EO) and transversus (TM) muscles in 14 patients with different degrees of airways obstruction (FEV1: 41 +/- 12%; FEV1/FVC: 45 +/- 10%; RV: 198 +/- 38%; PaO2: 75.8 +/- 12 y PaCO2: 41.4 +/- 5.7 mmHg). The EMG was obtained by insertion of bipolar electrodes guided by an ultrasound image of the abdominal wall to locate the position of the muscles. The measurements were recorded in supine decubitus position in 5 situations: a) breathing at tidal volume; b) slow expiration until RV; c) with inspiratory load; d) with expiratory load, and e) during relaxed breathing with the arms raised. Recordings were also made in the same situations with 10 patients sitting. Eight patients presented phasic expiratory activity during relaxed breathing (TM activity alone or accompanied by EO). We found no significant differences in degree of hyperinflation or in arterial gases between patients with phasic expiratory activity and those without. There were significant differences between these 2 groups, however, as to degree of airways obstruction, for absolute values of FEV1 (p < 0.02) and in raw values (p < 0.04). Slow breathing until RV recruited muscular activity in 13 patients; the muscles did not operate in unison, however, with TM acting first. Recruitment was also observed when inspiratory and expiratory loads were placed, although in this case the 3 muscles acted simultaneously. Phasic activity was observed in only 2 patients for recordings made with arms raised, at which time there was greater tonic muscle activity. The phasic activity pattern recorded when patients were sitting was very similar to that obtained in supine position. In summary, some patients with stable COPD have phasic expiratory activity of the abdominal muscles when resting. These muscles do not appear to act as a unit and this phasic expiratory activity is related to severity of upper airways obstruction.