Several cases of hyponatremia, with symptoms including altered mentation, seizures, and pulmonary edema, have been reported in endurance athletes over the last few years. This condition has been observed most frequently in individuals participating in ultra-distance events but has also been reported in a marathon runner. Excessive water intake has been identified as a common etiological factor. Given that this is a serious condition but that the risks of impaired performance and heat injury from inadequate water ingestion are substantial, how likely is it that hyponatremia will develop in events of marathon duration or shorter? A review of fluid and electrolyte balance--and an example of expected sodium and fluid losses in a marathon runner--suggest that normal regulatory mechanisms should prevent its occurrence in the absence of other contributing factors. Accordingly, ingestion of adequate fluid should remain a priority for individuals engaged in endurance activity.