Nitrous oxide (N O) is frequently used for short anaesthesia/analgesia in children undergoing painful or repetitive procedures . Children with acute lymphoblastic leukaemia (ALL) require repeated lumbar punctures with direct instillation of intrathecal chemotherapy, usually the anti-folate agent Methotrexate, during their treatment. These procedures are frequently performed under anaesthesia. Concerns have been intermittently raised about a drug-interaction between methotrexate and N O that may potentiate the undesirable side effects of methotrexate, including neurotoxicity. However, the clinical evidence consists mainly of isolated case reports leading to a lack of consensus amongst paediatric anaesthetists about the relative risk-benefits of using N O in children with ALL. In this article, we review the biochemical basis and scientific observations that suggest a significant interaction between N O and methotrexate due to their dual inhibition of the key enzyme methionine synthase. The possible role of this interaction in potentiating neurotoxicity in children with cancer is discussed, and arguments and counter-arguments about the clinical significance of this largely theoretical relationship are explored. Following comprehensive review of all the available data we make the case for the circumstantial evidence being sufficiently compelling to prompt a review of practice by paediatric anaesthetists and call for a precautionary approach by avoiding the use of N O in children receiving concurrent methotrexate.