Ultraendurance athletic events tax the limits of physiological homeostasis. Maintenance of sodium and water balance is a particularly difficult challenge in such events. We present the case of a 38-yr-old participant in the Bicycle Race Across America who developed severe pulmonary edema while cycling at an altitude of 2380 m on the fourth day of the race. With hospitalization and standard support for pulmonary edema, he made a quick, full recovery. A postrace work-up revealed no evidence of underlying cardiopulmonary disease or susceptibility to high-altitude pulmonary edema. His weight on the day of hospitalization was 2.7 kg greater than his prerace weight. We hypothesize that his excessive daily sodium intake (23-25 g, or 1000-1100 mEq) during the course of the race likely led to an expanded extracellular volume, increased hydrostatic pressure, and decreased oncotic pressure. These factors, in combination with ambient hypoxia, elevated cardiac output, and reduced renal perfusion expected with sustained, high-level exercise, may have led to the development of acute pulmonary edema. This case highlights the pitfalls of overly aggressive sodium intake in endurance races, particularly when such races are conducted at high altitude, where the hypoxia-induced rise in pulmonary artery pressures may amplify the effects of changes in hydrostatic and oncotic pressure that occur with extracellular volume expansion.