Osmotic demyelination syndrome following the rapid correction of chronic hyponatremia with combined water restriction and the administration of loop diuretics is the least known complication of chronic hyponatremia. We describe a 28-year-old woman who had complained of generalized weakness, anorexia, nausea, and dizziness for 3 days. The most striking biochemical abnormality was hyponatremia (Na+ 117 mmol/l). This hyponatremia met the laboratory diagnostic criterion for the syndrome of inappropriate secretion of antidiuretic hormone (SIADH). Treatment with 2 liters of normal saline for 2 days did not correct her hyponatremia. Therefore, she was treated with water restriction together and furosemide (40 mg per day). Polyuria and a body weight loss of 5 kg ensued, and her plasma Na+ concentration rose from 119 to 132 mmol/l within 24 h. Acute delirium, Wernicke’s aphasia, and parkinsonism developed 2 days later. Magnetic resonance imaging of the brain clearly showed typical features of extrapontine myelinolysis. Subsequent neurological and radiological features indicated various lesions of brain insult. We suggest that water restriction combined with the administration of loop diuretics might cause rapid correction of hyponatremia in some patients with SIADH. Calculation of the tonicity balance and close monitoring of serum and urine electrolytes are critical if osmotic demyelination syndrome is to be avoided during the correction of chronic hyponatremia.