To assess the pattern of abdominal muscle contraction in stable patients with chronic obstructive pulmonary disease (COPD), we studied electromyograms of the rectus abdominis, external oblique, and transversus abdominis muscles in 40 patients with variable degrees of chronic airflow obstruction (FEV1 between 17 and 82% of predicted); 12 control subjects with normal pulmonary function tests were studied for comparison. The subjects were studied during resting breathing in the supine posture, and the electromyograms were recorded with concentric needle electrodes implanted with the aid of a high-resolution ultrasound. The rectus abdominis and external oblique were silent in virtually all patients. In contrast, 17 patients had invariable phasic expiratory activity in the transversus abdominis, and 11 additional patients had intermittent transversus expiratory activity. Expiratory contraction of the transversus was related to the degree of airflow obstruction (p less than 0.005), and when present, it persisted in the seated posture. We conclude that (1) when breathing at rest, many stable patients with severe chronic airflow obstruction contract the abdominal muscles during expiration, and (2) this expiratory contraction is usually confined to the transversus muscle. These observations also indicate that the physiology of dynamic hyperinflation and intrinsic positive end-expiratory pressure (PEEP) in such patients should be reevaluated.