Methylnaltrexone, a peripheral opioid μ-receptor antagonist is licensed for subcutaneous administration for the treatment of severe opioid-induced constipation in adults. We describe the use of intravenous methylnaltrexone in a 3-year-old boy receiving a subcutaneous diamorphine infusion for palliation from widely metastatic alveolar rhabdomyosarcoma. The patient, who had not opened his bowels for 3 weeks despite use of regular conventional laxatives, was given a 150 mcg/kg dose via indwelling central venous catheter. Constipation was relieved within minutes of the injection. There were no side effects noted during or following injection, and no clinically apparent reduction in analgesia. Intravenous methylnaltrexone may provide a valuable additional treatment option in paediatric palliative care, especially for those with an oncological diagnosis, the majority of whom will have indwelling central venous access devices.