Disorders of serum sodium occur commonly in athletes participating in endurance sports. The most life-threatening of these is hyponatremia, which can occur in as many as 2% to 7% of participants. Exercise-associated hyponatremia (EAH) is caused by a combination of excessive water or hypotonic fluid intake as well as high levels of arginine vasopressin, which limits the ability of the kidney to excrete water. Other factors in the pathogenesis of EAH include sweat sodium loss, inability to mobilize exchangeable sodium stores, metabolic water production, and impaired renal blood flow and glomerular filtration rate. Most cases of EAH lead to minimal or absent complications and do not require specific therapy other than close monitoring and fluid restriction. However, a small number of athletes may present with severe and life-threatening hyponatremia associated with cerebral edema and possibly noncardiogenic pulmonary edema. Rapid diagnosis and appropriate therapy of these athletes with hypertonic saline is required to prevent severe complications or death.