To assess the risk of mortality in patients with hyponatremia at the time of hospital admission, the authors studied data for 13,979 patients admitted over a 46-month period. Of the 763 (4%) admitted with hyponatremia, 757 (99%) were matched by age, gender, and admitting date with normonatremic control patients. Hyponatremic patients were more than seven times as likely to die in the hospital than the control patients, and they were more than twice as likely to die after discharge (p less than 0.0001 for both). This relationship with in- and outpatient mortality held when controlling for the diagnoses found more often in the hyponatremic patients. Hyponatremia appears to be an indicator of increased risk of death regardless of the disease with which it is associated.