During thoracic surgery, one-lung ventilation (OLV) is associated with hypoxemia, lung injury, and perioperative respiratory complications. The level of positive-end expiratory pressure (PEEP) to apply during OLV remains controversial. The open-lung approach consists in setting a level of PEEP corresponding to the best lung compliance, using an esophageal catheter to measure the transpulmonary pressure. This approach has been effective in laparoscopic surgeries or acute respiratory distress syndrome, but has never been evaluated in thoracic surgery.