PURPOSE: To describe the incidence and etiology of hyponatremia in a cold weather ultraendurance event. METHODS: The subjects consisted of 16 athletes (8 cyclists, 8 runners) in the 100-mile Iditasport ultramarathon. Subjects were weighed and blood samples were drawn for measurement of serum sodium pre- and post-race. Race diets were analyzed to determine subjects' fluid and sodium consumption. Subjects were split by post-race serum sodium concentration into hyponatremic (plasma sodium concentration < 135 mmol/L) and normonatremic (plasma sodium concentration ≥ 135 mmol/L) groups for statistical analyses. RESULTS: Seven of 16 subjects (44%) were hyponatremic, including 3 male cyclists, 2 male runners, 1 female cyclist, and 1 female runner. The hyponatremic group was significantly lighter than the normonatremic group both pre- (−11.1 kg) and post-race (−10.0 kg). The hyponatremic group exhibited a significant decrease in serum sodium concentration (137.0 to 132.9 mmol/L) and the normonatremic group experienced a significant decrease in weight (−1.9 kg) pre- to postrace. The hyponatremic group drank more fluid per hour (+0.1 L/h) and consumed less sodium per hour (-63 mg) compared to the normonatremic group. CONCLUSION: Hyponatremia is common in an ultraendurance race held in the extreme cold, and may be caused by excessive fluid consumption and/or inadequate sodium intake.