Hyponatremia is the most frequent electrolyte disorder in critically ill neurological patients. The major differential diagnoses in this situation are the syndrome of inappropriate antidiuretic hormone secretion, marked by inappropriate retention of free water, and cerebral salt wasting, characterized by excessive urinary loss of sodium and resulting in polyuria and extracellular volume contraction. Cerebral salt wasting is a syndrome of hyponatremia due to increased urine output and excessive natriuresis described in patients with central nervous system disease. Although cerebral salt wasting has been well described in neurosurgical patients, data regarding pediatric patients is sparse. We present a 34-month-old boy with lissencephaly who developed cerebral salt wasting after brain biopsy. The patient was treated with hypertonic saline and multiple antiepileptic drugs. Fludrocortisone supplementation effectively treated cerebral salt wasting.