We describe cerebral salt wasting syndrome (CSWS) in a 5-year-old female child with Down syndrome who had acute myelopathy secondary to chronic atlantoaxial subluxation and fracture dislocation of the odontoid process. The patient developed hyponatraemia associated with excessive urine output and elevated urine sodium concentration following her injury. An administered volume-for-volume replacement of urine loss with 0.9% sodium chloride resulted in an excellent outcome. This patient illustrates the importance of ascertaining CSWS in children with spinal cord disorders, in addition to the syndrome of inappropriate antidiuretic hormone (SIADH) secretion and diabetes insipidus (DI) commonly encountered following a central nervous system (CNS) injury, as the specific treatment approaches is clearly associated with an excellent outcome.