Life-threatening hyponatremia can occur when sodium concentration falls to 125 mmol/L or less. Symptoms usually do not depend on the absolute sodium concentration but on the rate of fall. Estimates of mortality in acute hyponatremia are as high as 50%. Marathon runners are at particular risk of developing a syndrome which consists of severe hyponatremia, pulmonary edema and cerebral edema as originally described by Varon and Ayus. This syndrome, if not managed appropriately has a very high morbidity and mortality.