In the present study, we found that at least 4.4% of 1,088 prospectively studied patients developed postoperative hyponatremia (plasma sodium concentration less than 130 mEq/L). Most patients (42%) were normovolemic. Edematous states (21%), hyperglycemia (21%), volume depletion (8%), and renal failure (8%), however, were also common settings of postoperative hyponatremia. Plasma arginine vasopressin was present in all patients in whom it was measured and 94% of the patients were receiving hypotonic fluid at the time of development of hyponatremia. Hyponatremia was not associated with significant neurologic deterioration in the 48 postoperative patients in the present study. In eight patients, however, the positive water balance that resulted in hyponatremia was associated with development of pulmonary vascular congestion. We conclude that hyponatremia commonly occurs following all types of surgical procedures and is due to hypotonic fluid administration in the presence of nonosmotic secretion of arginine vasopressin.