With recent emphasis on increased water intake during exercise for the prevention of dehydration and exertional heat illness, there has been an increase in cases of hyponatremia related to excessive water intake. This article reviews several recent military cases and three deaths that have occurred as a result of overhydration, with resultant hyponatremia and cerebral edema. All of these cases are associated with more than 5 L (usually 10-20 L) of water intake during a period of a few hours. The importance of maintaining adequate hydration in exertional heat illness prevention cannot be overemphasized, but excessive fluid intake may lead to life-threatening hyponatremia. Current guidelines provide safety by limiting fluid intake during times of heavy sweating to 1 to 1.5 L per hour.