Background: N2O irreversibly inhibits the vitamin B12‐dependent enzyme, methionine synthase, and has the potential to cause hematological, neurological and cardiovascular adverse effects [1–5]. Repeated exposure to N2O is common in certain pediatric settings (burns dressing and radiotherapy). There are no published clinical studies investigating the effects of nitrous oxide anesthesia on vitamin B12 and folate metabolism in a pediatric population.Aims: To study the effect of repeated exposure to N2O on metabolic indices in a cohort of children predisposed to metabolic and nutritional disturbance.Method: In an on‐going, prospective study of children undergoing radiotherapy for cancer, homocysteine, methylmalonic acid, vitamin B12, folate and red cell indices were measured at regular intervals. Results were correlated with cumulative N2O exposure in those children who required repeated general anesthesia.Results: Forty children have been studied so far. Fifteen required general anesthesia to complete therapy. Median exposure to N2O was 28 min per exposure for an average of 13 exposures. The median cumulative exposure to N2O in this cohort was 397 min. Four children each had a cumulative exposure of more than 15 h. In these cases, nitrous oxide was delivered in 30 or more anesthetics over periods ranging from 6 to 11 weeks. Preliminary results show that homocysteine levels are not consistently correlated with N2O exposure. No clinical or biochemical adverse effects related to the gas have been detected.Conclusion: This interim analysis suggests that repeated N2O has a marginal effect on vitamin B12 metabolic indices in predisposed children. The study is on‐going.This study is supported by a grant from SPANZA.References 1 Nunn JF. Clinical aspects of the interaction between nitrous oxide and vitamin B12. British Journal of Anaesthesia 1987; 59:3‐132 Myles PS, Leslie K, Chan MTV, Paech MJ, Peyton P, Pascoe E. Avoidance of nitrous oxide for patients undergoing major surgery: a randomised controlled trial. Anesthesiology 2007; 107:221‐231.3 Nagele P, Zeugswetter B, Wiener C, Burger H, Hupfl M, Mittlbock M et al. Influence of methylenetetrahydrofolate reductase gene polymorphism on homocysteine concentration after nitrous oxide anesthesia. Anesthesiology 2008; 109:36‐43.4 Badner NH, Beattie WS, Freeman D, Spence JD. Nitrous oxide induced increased homocysteine concentrations are associated with increased postoperative myocardial ischemia in patients undergoing carotid endarterectomy. Anesthesia and Analgesia 2000; 91:1073‐1079.5 Badner NH, Drader K, Freenab D, Spence JD. The use of intraoperative nitrous oxide leads to postoperative increases in plasma homocysteine. Anesthesia and Analgesia 1998; 87:711‐713