Correction of hyponatremia can be complicated by brisk free water diuresis with a rise in the serum sodium (s-Na) in excess of the generally accepted rate of 10-15 mmol/l/24 hours. We describe this complication and its treatment with desmopressin (dD-AVP), in a 56-year-old female with severe hyponatremia secondary to polydipsia and antidiuretic (ADH) activity. The patient developed a large free water diuresis with a markedly dilute urine (urine osmolality 61 mmol/kg) and a rise in the serum sodium of 19 mmol/l in 19 hours despite the addition of large volumes of free water intravenously and orally. To reduce the free water excretion, desmopressin (dD-AVP) 8 microg was given intravenously. This resulted in a rise in the urinary osmolality, a reduction in the urine volume, and a 2 mmol/l reduction in the serum sodium. Thereafter, the serum sodium rose 4 mmol/l in 24 hours. There were no neurological sequellae. In cases of appropriate but rapid correction of hyponatremia secondary to rapid free water diuresis, dD-AVP can safely reduce the free water excretion, slow the rate of correction of the serum sodium and simplify the fluid therapy of the patient.