Children undergoing major craniofacial surgery develop a varying degree of perioperative metabolic acidosis persisting for several hours. The maximum BD appears to be related to the amount of intraoperative blood loss and replacement rather than duration of surgery. As it is difficult to predict the extent and duration of metabolic acidosis for an individual patient, this study confirmed our current practice that all patients should be admitted to a neurosurgical high-dependency unit postoperatively for overnight monitoring.