We assessed the incidence and etiology of hyponatremia in the 100-mile (161 km) Iditasport ultramarathon. Subjects (8 cyclists, 8 runners) were weighed and serum sodium was measured pre- and post-race. Race diets were analyzed to determine fluid and sodium consumption. Subjects were split by post-race serum sodium concentration into hyponatremic and normonatremic groups for statistical analyses. Seven of 16 subjects (44%) were hyponatremic. 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 (82.1 to 80.2 kg) pre- to post-race. The hyponatremic group drank more fluid per hour (0.5 versus 0.4 L/h) and consumed less sodium per hour (235 versus 298 mg/h) compared to the normonatremic group. In 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.