The bulk of clinical and experimental evidence seems to support the view that a catastrophic outcome can accompany rapid correction of chronic hyponatremia. However, the dilemma that faces the clinicians is controlling the rate at which the serum sodium concentration is increased during the treatment of hyponatremia. We present a case of severe hyponatremia and hypokalemia to illustrate, in a quantitative fashion, that the increase in serum sodium concentration will be determined not only by the tonicity of infused fluids, but also, and to a major degree, by the rate of excretion of free water by the kidney. A rapid increase in serum sodium concentration would occur if antidiuretic hormone (ADH) release is inhibited and water diuresis ensues. We suggest that the administration of potassium chloride (orally or via a central line) would have been the appropriate therapy for this patient.