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General anesthesia promotes atelectasis formation, which is augmented by administration of large oxygen concentrations. We studied the efficacy of positive end-expiratory pressure (PEEP) application during the induction of general anesthesia (fraction of inspired oxygen [Fio2] 1.0) to prevent atelectasis. Sixteen adult patients were randomly assigned to one of two groups. Both groups breathed 100% O2 for 5 min and, after a general anesthesia induction, mechanical ventilation via a face mask with a Fio2 of 1.0 for another 5 min before endotracheal intubation. Patients in the first group (PEEP group) had continuous positive airway pressure (CPAP) (6 cm H2O) and mechanical ventilation via a face mask with a PEEP of 6 cm H2O. No CPAP or PEEP was applied in the control group. Atelectasis, determined by computed radiograph tomography, and analysis of blood gases were measured twice: before the beginning of anesthesia and directly after the intubation. There was no difference between groups before the anesthesia induction. After endotracheal intubation, patients in the control group showed an increase of the mean area of atelectasis from 0.8% ± 0.9% to 4.1% ± 2.0% (P = 0.0002), whereas the patients of the PEEP group showed no change (0.5% ± 0.6% versus 0.4% ± 0.7%). After the intubation with a Fio2 of 1.0, Pao2 was significantly higher in the PEEP group than in the control (591 ± 54 mm Hg versus 457 ± 99 mm Hg; P = 0.005). Atelectasis formation is prevented by application of PEEP during the anesthesia induction despite the use of large oxygen concentrations, resulting in improved oxygenation.