The charts were reviewed of children admitted in diabetic ketoacidosis (DKA) to one hospital within 12 years. The frequency of brain herniation after admission was nine of 153 children admitted for one or more episodes of DKA. The severity of acidosis and hypercapnea were the most reliable risk factors. None of the children who maintained a blood pH greater than 7.1 and a capillary blood partial pressure of carbon dioxide (PCO2) greater than 20 mm Hg manifested brain herniation. The rate of initial fluid administration in severe DKA was also a risk factor. Of 119 patients having a blood pH less than 7.1 or PCO2 less than 20 mm Hg, none of 32 receiving less than 25 mL/kg, one of 42 receiving 25-50 mL/kg, and eight of 40 receiving more than 50 mL/kg of intravenous fluid during the first (in Patient 9, the second) 4 hours of therapy sustained brain herniation. Equally dehydrated unaffected patients initially receiving 25-50 mL/kg/4 hours of intravenous fluid did not develop signs of hypovolemia or worsening DKA. In this series, hydrating at a rate greater than 50 mL/kg during the first 4 hours offered no advantage and was associated with an increased risk of brain herniation.