To determine the effects of mild hypothermia therapy (34 degrees C) for brain edema caused by hypoxic ischemic encephalopathy (HIE) or acute encephalitis/encephalopathy, we reviewed the charts and serial brain CT images in six children (males 3, average age 1.6 years) treated with mild hypothermia therapy between November 2006 and April 2009. Both of the two children with HIE after cardiopulmonary arrest did not show any deterioration of brain edema after the initiation of hypothermia therapy. However, two of four non-HIE patients (acute encephalitis/encephalopathy 3 cases and metabolic encephalopathy plus HIE 1 case) showed progressive brain edema during the cooling phase and re-warming phase, respectively. There were no differences between patients with and those without progressive brain edema with regard to the interval until initiation of mild hypothermia therapy, duration of cooling phase, duration of re-warming phase, or peak serum NSE (neuron-specific enolase) levels. However, two children with progressive brain edema showed a delayed NSE peak time (15 and 13 days after onset, respectively), compared with those without progressive brain edema (2-6 days after onset). Our study suggests that serial measurement of serum NSE might be useful marker for adjusting the methods of hypothermia therapy according to neuropathology. Further study is necessary to establish optimal hypothermia therapy especially in children with acute encephalitis/ encephalopathy.