Hyponatremia is the most common electrolyte abnormality in clinical practice. Often overlooked or misdiagnosed, hyponatremia can quickly deteriorate and cause serious and potentially life-threatening complications. In the orthopedic patient, hyponatremia can mimic common postoperative sequelae, thereby making diagnosis even more difficult. Although serious complications of hyponatremia are rare, they are severe, and early detection is crucial in initiating prompt treatment. From 2008 to 2010, 392 consecutive hip and knee arthroplasties were prospectively monitored for development of hyponatremia. Hyponatremia occurred in 155 (40%) of cases. Although the majority were mild, 7% were moderate or severe deficiencies, which are associated with increased morbidity. Risk factors included preoperative hyponatremia, female sex, older age, lower body weight, knee more than hip surgery, and bilateral knee arthroplasty. Thiazides and angiotensin-converting enzyme inhibitors were the only medications associated with low postoperative sodium values. Hyponatremia is common after elective orthopedic surgery and is associated with identifiable perioperative risk factors. Although the most severe complications are rare, failure to identify mild cases of hyponatremia allows the sodium deficit to worsen unnecessarily. Early detection is important because, unlike severe cases, mild hyponatremia is generally treatable with simple medical management.