To determine the frequency, causes, and clinical significance of hyponatremia in hospitalized children, we reviewed the clinical and laboratory data of all hyponatremic children in Texas Children's Hospital over a 12-month period. One hundred sixty-one patients from among 11,702 hospital admissions were found to have hyponatremia, defined as serum sodium value of less than 130 mEq/L, an overall frequency of 1.38%. Sixty-nine patients (43%) had hyponatremia on admission, and 92 patients (57%) had hospital-acquired hyponatremia. Thirty-seven (23%) were previously healthy children, and 124 (77%) had chronic illnesses. Acute gastroenteritis was the leading cause of hyponatremia present on admission, and diuretic therapy was the leading cause of hospital-acquired hyponatremia. Only four patients (2.5%) had a serum sodium concentration of less than 120 mEq/L. Six patients (3.7%) had neurologic impairment on discharge, and 19 patients (12%) ultimately died long after their hyponatremia was corrected. Each patient who had neurologic sequelae and each patient who died had underlying medical conditions which could explain their morbidity and/or mortality. The prognosis appears to be more clearly related to the underlying medical disorder rather than to the hyponatremic state or its correction.