This study of seventeen patients presenting to the emergency room with acute severe asthma determined levels of antidiuretic hormone in serum as well as serum electrolytes and arterial blood gases serially. There was a progressive increase in plasma ADH concentration with severe asthma and ADH levels were substantially higher in those patients with PaCO2's higher than 45 Torr. Patients treated with intravenous aminophylline had a fall in ADH levels while those treated with injectable epinephrine showed an elevation of these levels. As the asthma improved, ADH levels decreased in all patients. In this series of children, sick for not more than 24 hours, serum sodium levels were normal. However, children who have been ill for longer periods than this or who are on prolonged intravenous fluid therapy need close monitoring of serum electrolyte concentrations.