Objectives: The purpose of this study was to investigate the effects of PEEP on oxygenation and airway pressures during PCV-OLV. Design: Randomized, crossover, clinical study. Setting: University hospital. Participants: Twenty-five patients undergoing thoracotomy. Interventions: During the first 5 minutes of OLV, all patients were ventilated with VCV (PEEP: 0) (VCV-ZEEP). Afterward, ventilation was changed to PCV with PEEP: 0 (PCV-ZEEP) or PEEP: 4 cmH(2)O (PCV-PEEP) for 20 minutes. In the following 20 minutes, PCV-PEEP and PCV-ZEEP were applied in reverse sequence. Measurements and Main Results: At the end of VCV-ZEEP airway pressures (peak airway pressure, plateau airway pressure, mean airway pressure, and pause airway pressure) were recorded. At the end of PCV-PEEP and PCV-ZEEP air-way pressures, PaO2 and Qs/Qt were recorded. Ppeak and Pplat were significantly lower with PCV-PEEP compared with VCV-ZEEP (eg, Ppeak: 33.4 +/- 4.2,28.3 +/- 4.1, and 28.9 +/- 3.7 cmH(2)O in VCV-ZEEP, PCV-ZEEP, and PCV-PEEP, respectively; p < 0.05 for PCV-ZEEP v VCV-ZEEP and PCV-PEEP v VCV-ZEEP). PCV-PEEP was associated with an increased PaO2 (230.3 +/- 69.8 v 189.0 +/- 54.8 mmHg, p < 0.05) and decreased Qs/Qt (33.4% +/- 7.3% v 38.4% +/- 5.7%, p < 0.05) compared with PCV-ZEEP (mean SD). Eighty-eight percent of the patients have benefited from PEEP. Conclusion: During OLV, PCV with a low level of PEEP leads to improved oxygenation with lower airway pressures. (C) 2005 Elsevier Inc. All rights reserved.