In brief: Hyponatremia is known to occur in athletes competing in endurance running events greater than marathon distances; it has also stricken participants in the Ironman Triathalon. This article describes a case of hyponatremia associated with altered mental status, increased intracranial pressure, and pulmonary edema in a 21-year-old man who had completed his first marathon. Salt loss from sweat and retention of hypotonic fluids are the most likely causes. This is the first reported case of this syndrome occurring at a marathon distance.