Although the syndrome of inappropriate antidiuretic hormone secretion (SIADH) is commonly observed in patients with acute or chronic central nervous system (CNS) disorders, cerebral salt wasting (CSW) that results in hyponatremia has rarely been reported in children. Both SIADH and CSW result in increased urinary sodium excretion and hyponatremia. However, the management protocols for these two conditions are quite different; volume restriction is used in treating SIADH, while volume expansion is necessary for the treatment (of CSW. We present a case of CSW in a child with cervicothoracic hematoma secondary to head and cervicothoracic trauma, without evidence of brain edema. The child was diagnosed on the basis of high urinary sodium excretion resulting in hyponatremia and low serum osmolarity. Improvements in serum sodium levels after saline hydration confirmed this diagnosis. We believe that potentially dangerous cases of hyponatremia should be carefully evaluated in children with cervicothoracic hematoma secondary to trauma, including situations in which brain edema is absent.