Pseudohyponatremia is a falsely low serum sodium measurement. It occurs in cases of extreme hyperlipidemia or hyperproteinemia when serum sodium is measured by some--but not all--laboratory methods. This article reviews the most common techniques for measuring serum sodium levels, explains why pseudohyponatremia occurs, and identifies specific situations in which pseudohyponatremia can lead to dangerous errors in patient management. The review describes the dramatic change in prevalence of the different laboratory methods for measuring serum sodium over the past decade, and emphasizes the need for clinicians to be familiar with the methods of their clinical laboratory in order to properly interpret a reported serum sodium determination. I offer recommendations for the rational use of the different laboratory methods in various clinical situations.