Depending upon the age and sex of a human, water constitutes 55% to 80% of the body's weight and provides a milieu vital for survival. Water imbalance is common among the critically ill. Excessive increases or decreases in body water can be lethal. There are numerous pathologic and iatrogenic causes for water imbalance, the most troublesome being disorders of antidiuretic hormone (ADH) secretion. Antidiuretic hormone plays a pivotal role in conserving water by increasing reabsorption of water by the kidney. Without the influence of ADH (as is seen in diabetes insipidus), a person would be required to ingest between 5 and 15 L of water daily to match urinary losses. Conversely, excessive ADH secretion would reduce urine output in adults to as little as 500 mL per day, dangerously diluting blood volume and expanding intracellular volume. This is what causes the symptoms of the syndrome of inappropriate ADH (SIADH). The care of patients who are critically ill and have disorders of ADH secretion can be challenging. The challenge lies in the recognition and treatment of the disorder. A collaborative team approach helps patients achieve and maintain the delicate balance of body fluids.