Since its first description in 1985, two opposing theories have evolved to explain the etiology of symptomatic hyponatremia of exercise. The first holds that the condition occurs only in athletes who lose both water and sodium during exercise, and fail to fully replace their sodium losses. The second theory holds that the symptomatic form of this condition occurs in athletes who generate a whole body fluid overload as a result of an excessive fluid intake during prolonged exercise. It is argued that the promotion of the idea that athletes should drink as much as possible during exercise has produced, rather than prevented, the recent increase in the incidence of this condition. A series of case reports and laboratory studies reported in the past 2 years have established that it is a whole body fluid overload, resulting from sustained high rates of fluid intake, that causes the symptomatic hyponatremia of exercise. There is no evidence that, in the absence of fluid overload, the usual sodium deficits generated during exercise can cause this condition. These findings confirm that the potentially fatal condition of symptomatic hyponatremia would be eliminated from sport immediately if all athletes were advised of the dangers of ingesting as much fluid as possible during any exercise that lasts more than 4 hours.