Methylnaltrexone, a peripherally acting µ-opioid receptor antagonist, has been studied in adults for the treatment of opioid-induced constipation in advanced illness. Here, the authors document the first neonate to receive methylnaltrexone in an attempt to resolve morphine-induced urinary retention. An asphyxiated term newborn infant underwent induced hypothermia and received morphine by continuous intravenous infusion. After 36 h, the patient developed progressive urinary retention (calculated bladder volume 63 ml), followed by venous congestion of the lower extremities. Attempted bladder catheterisation was unsuccessful. Voiding occurred within 20 min after intravenous administration of methylnaltrexone (0.15 mg/kg body weight). A relapse of urinary retention 24 h later responded well to a second dose of methylnaltrexone. There were no adverse effects and no opioid withdrawal symptoms. The neonate had normal findings in cranial MRI that was performed after elective cessation of induced hypothermia.