Laboratories are required to have a critical values policy as a patient safety measure. Serum sodium commonly is included in critical results lists, but a wide range of values are used. We studied all critical serum and whole blood sodium results called to clinicians during a 6-month period. Patients' electronic medical records were reviewed for clinical responses and patient outcomes. Of the 111,545 sodium results occurring during the study, 615 (0.6%) were critical. By using criteria of 120 mEq/L (120 mmol/L) or less and 155 mEq/L (155 mmol/L) or more, we found 166 critically low results and 447 critically high results. In hypernatremic and hyponatremic patients, the lengths of stay were increased above our average, and clinicians responded to more than 50% of results within 4 hours. The mortality rates of hyponatremic and hypernatremic inpatients were 19% and 48%, respectively. Disease severity as measured by length of stay and mortality indicated these critical limits should not be broadened.