In this issue of the Archives, Leung and colleagues1 present an interesting and important epidemiological analysis of almost 1 million patients who underwent surgery between January 1, 2005, and December 31, 2010, using the American College of Surgeons National Surgical Quality Improvement Program (ACS NSQIP), a validated database with nearly 300 participating academic and community hospitals. They demonstrate that preoperative hyponatremia compared with a normal preoperative serum sodium level is significantly associated with a higher risk of perioperative 30-day mortality (adjusted odds ratio [aOR], 1.44) and increased morbidity (major coronary events [aOR, 1.21], wound infections [1.24], pneumonia [1.17], and approximately 1 additional adjusted median hospital day). Preoperative hyponatremia was common, defined as a serum sodium level less than 135 mEq/L (to convert to millimoles per liter, multiply by 1.0) within 90 days of surgery, occurring in 7.8% of patients, or 75 423 of 964 263. In addition, almost 89% of hyponatremia cases were mild, defined as a serum sodium level of 130 to 134 mEq/L. Although, as expected, the risk of death was directly related to the severity of the hyponatremia, somewhat surprisingly, nonemergency procedures and low-risk cases (as stratified by American Society of Anesthesia classes 1 and 2) were significantly associated with higher risk (aOR, 1.59 and 1.93, respectively). Increased risk in patients scheduled for elective surgery with mild and almost certainly asymptomatic hyponatremia presents an important opportunity for the internist performing preoperative medical consultation.