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: Background Controversy still exists if the administration of supplementary oxygen to patients having emergency Caesarean section (CS) under regional anaesthesia is beneficial or potentially harmful. Therefore, in a prospective double-blinded study, we randomized patients having emergency CS under regional anaesthesia to receive either air or 60% oxygen until delivery and compared the effects on fetal oxygenation and lipid-peroxidation in the mother and baby. : Methods We recruited 131 women having emergency CS under regional anaesthesia. Either 21% (air group) or 60% oxygen (oxygen group) was administered using a Venturi-type facemask until delivery. We compared the oxygen exposure duration, umbilical arterial (UA) and venous (UV) blood gases and oxygen content, and plasma concentration of 8-isoprostane. Subanalysis was performed according to whether or not fetal compromise was considered present. : Results Data from 125 patients were analysed. For the oxygen group vs the air group, there were greater values for UA Po2 [mean 2.2 (sd 0.5) vs 1.9 (0.6) kPa, P=0.01], UA O2 content [6.6 (2.5) vs 4.9 (2.8) ml dl−1, P=0.006], UV Po2 [3.8 (0.8) vs 3.2 (0.8) kPa, P<0.0001], and UV O2 content [12.9 (3.5) vs 10.4 (3.8) ml dl−1, P=0.001]. There was no difference between the groups in maternal, UA, or UV 8-isoprostane concentration. Apgar scores and UA pH were similar between the groups. Similar changes were observed regardless of whether fetal compromise was considered present (n=37) or not (n=88). : Conclusions Breathing 60% oxygen during emergency CS under regional anaesthesia increased fetal oxygenation with no associated increase in lipid-peroxidation in the mother or fetus. [ABSTRACT FROM AUTHOR]