Background: Although high inspired oxygen fraction (FiO 2) is common practice in pediatric anaesthesia, there is lack of evidence for affects on postoperative lung function. Therefore, we compared postoperative changes in lung volume, ventilation heterogeneity and respiratory mechanics while ventilating anaesthetised children with low or high FiO 2. Methods: In a double-blinded, randomized, controlled trial, children scheduled for elective surgery were randomly assigned 100% FiO 2 (Group H) or 80% FiO 2 (Group L) during anaesthesia induction and emergence, while anaesthesia was maintained with either FiO 2 of 80% (Group H) or 35% (Group L). During spontaneous breathing, we measured functional residual capacity (FRC) and lung clearance index (LCI) by multiple-breath nitrogen washout, airway resistance and respiratory tissue elastance by forced oscillations preoperatively (M1), after discharge from the recovery room (M2) and at day 1 postoperatively (M3). Secondary outcomes were the occurrence of respiratory complications, post-operative nausea and vomiting and surgical site infection at 30 days postoperatively. Results: FRC decreased in Group H at M2 (-14.7% [-21.0,-8.3] 95% CI, p=0.008) but normalized at M3. Ventilation inhomogeneity (LCI) increased in both groups at M2 but remained elevated only in Group H at M3 (6.3% [-0.3, 12.9%], p=0.023). No differences in respiratory mechanical parameters or in secondary outcomes were observed. Conclusions: High FiO 2 decreases lung volume in the immediate postoperative period with long-lasting enhancement in lung heterogeneity. This result, in addition to the wellestablished harmful effects of hyperoxia, supports the avoidance of high FiO 2 during anaesthesia management of children with normal lungs.