Using a special gastroesophageal catheter, electromyographic measurements of the diaphragm (Edi) and transdiaphragmatic pressure (Pdi) were taken in the supine, standing, erect sitting, and leaning forward (sitting) positions in 8 normal subjects and 6 patients with severe chronic obstructive pulmonary disease (COPD) with marked hyperinflation and low fat diaphragms. Four patients had pronounced postural relief of their dyspnea from assuming the supine and/or leaning forward positions. All 8 normal subjects and 4 of the 6 patients with COPD showed substantial (2- to 5-fold) increases in delta Edi, the phasic inspiratory amplitude of Edi, on assuming the standing and erect sitting postures. In the normal subjects, delta Pdi, the phasic inspiratory increment in Pdi, was maintained in all 4 postures, whereas in all patients with COPD, it decreased significantly in the erect sitting and standing postures. In 2 of the 6 patients with COPD, the delta Edi did not increase in the erect postures. This suggested that in these patients a reflex, which normally compensates for reduced diaphragmatic efficiency because of shortened muscle fibers in the erect postures, was not operating. Although it is not clear what mechanism(s) might account for suppression of this compensatory reflex, such reflex suppression might be advantageous from the viewpoint of diaphragmatic muscle energetics. The diaphragm would be thus spared energetically wasteful attempts to accomplish a fatiguing mechanical task.