Central pontine and extrapontine myelinolysis are uncommon disorders characterized by distinctive clinical features and typical findings on neuroimaging. Only a few cases are reported in the pediatric age group. We describe the case of a leukemic, malnourished 14-year-old boy with a high serum sodium concentration that gradually increased to 170 mmol/L. During a septic shock episode, hydration with a low sodium concentration at the rate of 104 mL/h for 24 h was administered. A rapid correction of the high serum sodium occurred, exceeding 0.5 mmol/L/h. The following day the patient developed rapid and progressive neurological impairment with clinical features characteristic of central pontine and extrapontine myelinolysis. Magnetic resonance imaging confirmed the diagnosis 11 days later. The patient was treated with steroids and intravenous immunoglobulins. He achieved an almost full neurological recovery and radiological improvement. The reported case demonstrates that central pontine and extrapontine myelinolysis can occur after excessively rapid correction of hypernatremia.