Patients in status asthmaticus often have elevated plasma antidiuretic hormone levels. To determine if children in status asthmaticus have impaired water excretion and an increased risk of developing significant hyponatremia when given a fluid challenge, five consecutive patients who showed moderate asthmatic symptoms after taking two doses of epinephrine hydrochloride were given a fluid challenge (20 mL/kg of 5% dextrose in 0.2% normal saline solution given intravenously over 30 minutes followed by maintenance fluids [1,500 mL/sq m/24 hr] for 50 minutes). Urine was collected at 20-minute intervals for measurement of free-water clearance and percent water-load excretion in 80 minutes. This protocol was repeated 24 to 48 hours later, after clinical improvement. None of the patients was hyponatremic during status asthmaticus before water loading. However, four of five patients were mildly hyponatremic (serum sodium level between 130 and 132 mEq/L) between status asthmaticus and after clinical improvement. These same four patients also became mildly hyponatremic after fluid challenge during status asthmaticus. Maximal free-water clearance and percent water load excretion in 80 minutes were significantly lower during status asthmaticus after fluid challenge compared with results obtained after water loading when the patients' conditions were clinically improved. We conclude that patients in status asthmaticus have impaired water excretion after water loading but with a small risk of significant hyponatremia; a patient remaining in status asthmaticus and given large volumes of hypotonic fluid over a prolonged period of time may be at higher risk for significant hyponatremia.