OBJECTIVE Evaluation of the magnitude of pulmonary air trapping during routine thoracic surgery and single-lung transplantation. DESIGN Prospective study on consecutive patients. SETTING Single institution, university hospital. PARTICIPANTS Sixteen patients with no or moderate obstructive lung disease undergoing routine thoracic surgery (group 1), six patients with severe emphysema (group 2), and six patients with severe fibrosis (group 3) undergoing single-lung transplantation. INTERVENTIONS Occlusion maneuver timed at the end of expiration to measure auto-positive end-expiratory pressure (auto-PEEP) and trapped volume (delta FRC). The maneuver was performed during two-lung ventilation in supine (2LV supine) and lateral decubitus (2LV lateral) positions and during one-lung ventilation (OLV) in lateral decubitus position. At the same time, airway pressures and PaO2 measurements were performed. MEASUREMENTS AND MAIN RESULTS In group 1, consistent values of auto-PEEP and delta FRC occurred only during OLV: 4.8 +/- 2.5 cm H2O and 109 +/- 61 mL (mean +/- standard deviation). In group 2, auto-PEEP and delta FRC values were 11.7 +/- 6.9 cm H2O and 355 +/- 125 mL during 2LV supine, 8.8 +/- 5.7 cm H2O and 320 +/- 122 mL during 2LV lateral, and 15.9 +/- 3.9 cm H2O and 284 +/- 45 mL during OLV. In group 3, pulmonary air trapping was low. For the three groups together, auto-PEEP and delta FRC (p < 0.0001) related inversely to the ratio of forced expired volume in 1 second (FEV1) to forced vital capacity (FVC) expressed in percent (FEV1/FVC%) during OLV. In contrast, there was no correlation between PaO2 and auto-PEEP or delta FRC. CONCLUSION Pulmonary air trapping must be suspected in patients with no or moderate obstructive lung disease during OLV and in those with severe obstructive disease as soon as 2LV is initiated.