The study was undertaken to evaluate the role of coordination between the chest wall and abdomen during exercise in patients with chronic obstructive pulmonary disease (COPD). There were 40 patients with COPD and 6 control subjects with normal lung function who underwent a progressive exercise stress test on a treadmill ergometer. The normal subjects exhibited symmetrical motion between the chest wall and abdomen. Three separate patient groups were differentiated by differences in abdominal response to increasing exercise. Group I was similar to normal or showed an early abdominal peak. Group II had a prolonged outward motion of the abdomen, and Group III had an inward motion of the abdomen during inspiration. Resting pulmonary function (FEV1, VC, DL, RV/TLC) and exercise response (duration, O2 saturation, and maximal VO2) were progressively more abnormal from Group I through Group III. The addition of oxygen to Group III had no effect on the pattern observed. However, when 2 patients with a Group III response were reexercised flexed 45 degrees at the waist they no longer were completely paradoxical, they were less dyspneic, and they could walk farther. It is concluded that the chest-abdominal coordination is related to the underlying pulmonary abnormality, and the paradoxical pattern seen in some patients (Group III) is associated with very severe exercise limitation.