Diuretic therapy and administration of IV hypotonic fluids (dextrose in water or half-normal saline) form very important and frequent etiologies for hyponatremia, either as the sole agent or superimposed on some condition predisposing to hyponatremia. In several studies of patients with hyponatremia, diuretic use was considered to be the major contributing factor or sole etiology in about 30% of cases (range, 7.6%-46%). In two series of patients with severe hyponatremia (serum sodium <120 mEq/L), diuretics were implicated in 30%-73% of cases. Hyponatremia due to diuretics without any predisposing or contributing factors is limited mostly to patients over the age of 55 years. IV fluid administration is less often the sole cause for hyponatremia (although it occurs) but is a frequent contributing factor. In one study of postoperative hyponatremia, 94% of the patients were receiving hypotonic fluids. If renal water excretion is impaired, normal maintenance fluid quantities may lead to dilution, whereas excessive infusions may produce actual water intoxication or pulmonary edema. There may also be problems when excessive losses of fluid or various electrolytes occur for any reason and replacement therapy is attempted but either is not adequate or is excessive. The net result of any of the situations mentioned is a fluid disorder with or without an electrolyte problem that must be carefully and logically reasoned out, beginning from the primary deficit (the cause of which may still be active) and proceeding through subsequent events. Adequate records of fluid and electrolyte administration are valuable in solving the problem. In nonhospitalized persons a similar picture may be produced by dehydration with conscious or unconscious attempts at therapy by the patient or relatives. For example, marked sweating leads to thirst, but ingestion of large quantities of water alone dilutes body fluid sodium, already depleted, even further. A baby with diarrhea may be treated at home with water or sugar water; this replaces water but does not adequately replace electrolytes and so has the same dilutional effect as in the preceding example. On the other hand, the infant may be given boiled skimmed milk or soup, which are high-sodium preparations; the result may be hypernatremia if fluid intake is not adequate.