In 14 acutely ill patients with chronic obstructive pulmonary disease who were either intubated or breathed via a tracheostomy, we measured the pressures generated in the esophagus and trachea during the first 0.1 s of spontaneous inspiratory efforts against closed airway (P0.1). No significant difference was found between P0.1 in the trachea and esophagus, where it was measured at 2 balloon levels (distance from balloon tip to nares: 35 and 45 cm). In 4 of the patients we also compared esophageal and mouth occlusion pressures during mouth breathing, the orifice of tracheostomy being temporarily obstructed. Mouth P0.1 in the 4 patients averaged approximately 47% of the esophageal occlusion pressure (4.4 +/- 1.7 versus 8.9 +/- 3.1 cm H2O, respectively). Because in patients with COPD with upper airways bypassed (tracheostomized or intubated) the changes in esophageal and tracheal pressure during occluded respiratory efforts were similar, it is concluded that the difference between esophageal and mouth occlusion pressure is due to the tissue compliance of the oropharynx.