BACKGROUND: Supplemental oxygen has been reported to diminish postoperative nausea and vomiting (PONV). Surgical trauma causes increased response of stress hormones. Therefore, we aimed to investigate whether supplemental oxygen attenuates release of adrenocorticotrophic hormone (ACTH) and cortisol as well as PONV in patients undergoing thyroidectomy. METHODS: One hundred female patients were randomly assigned to two groups: 30% oxygen (Group 30) and 80% oxygen (Group 80). The incidence and the severity of PONV and pain score were evaluated 2, 6, 24 h postoperatively. PaO2, ACTH and cortisol were measured in 40 patients before tracheal intubation under 100% oxygen and at the end of surgery under designated oxygen concentration. RESULTS: The PaO2 at induction was similar between the groups, but significantly higher in the group 80 than group 30 at the end of surgery. There were no differences in the incidence and the severity of PONV and pain score postoperatively between the groups. ACTH increased significantly at the end of surgery in both groups, but cortisol did not. There were no differences in ACTH and cortisol between the groups. CONCLUSIONS: Supplemental oxygen during thyroidectomy did not reduce the incidence and severity of PONV, postoperative pain, and stress hormone responses.