UNLABELLED: BACKGROUND AND METHDS: Alveolar recruitment maneuver (ARM) is indicated in the treatment of intraoperative atelectasis. The objective of the present study was to compare two techniques of ARM using the response of the PaO2/FiO2 ratio and [PaO2 + PaCO2] in patients with grade III obesity. METHODS: This was an open prospective study with adult patients with grade III obesity who underwent bariatric surgery under volume-controlled mechanical ventilation with positive end-expiratory pressure (PEEP) of 5 cmH2O, divided in three groups: G CONT: PEEP of 5 cmH2O; G ARM10/15/20 after suture of the aponeurosis: progressive increase in PEEP to 10, 15, and 20 cmH2O with a 40-second pause and maintaining each level of PEEP for 2 minutes; and G ARM30 after suture of the aponeurosis: sudden increase in PEEP to 30 cmH2O with a 40-second pause and maintaining a PEEP of 30 for 2 minutes. Heart rate, mean arterial pressure, systolic and diastolic blood pressure, mean (P AW) and plateau (P PLAT) airways pressure, partial pressure of oxygen (PaO2), partial pressure of carbon dioxide (PaCO2), PaO2/FiO2 ratio (inspired fraction of oxygen), and [PaO2 + PaCO2] were analyzed. RESULTS: The following parameters showed statistically significant differences among the study groups: P PLAT, P AW, PaO2, PaO2/FiO2 ratio, and [PaO2 + PaCO2] (p < 0.0001). Comparing the groups two by two, the following parameters showed statistically significant differences: for P PLAT and P AW: G CONT x G2ARM10/15/20 and G CONT x G ARM30; and for PaO2/FiO2 ratio and [PaO2 + PaCO2]: G CONT x G ARM30. CONCLUSIONS: Alveolar recruitment maneuver with sudden increase of PEEP to 30 cmH2O showed a better response of the PaO2/FiO2 ratio.