Hyponatremia is characterized as either "true hyponatremia," which represents a decrease in the Na(+) concentration in the water phase of plasma, or "pseudohyponatremia," which is due to an increased percentage of protein or lipid in plasma, with a normal plasma water Na(+) concentration ([Na(+)]). Pseudohyponatremia is a known complication of intravenous immunoglobulin (IVIG). Because IVIG has been reported to result in post-infusional hyperproteinemia, IVIG-induced hyponatremia has been attributed to pseudohyponatremia. In this case report, we demonstrate that IVIG therapy can result in true hyponatremia, resulting from sucrose-induced translocation of water from the intracellular compartment (ICF) to the extracellular compartment (ECF), as well as the infusion of a large volume of dilute fluid, in patients with an underlying defect in urinary free water excretion.