Objective : Acute cerebral infarction is often accompanied by transtentorial herniation which can be fatal. The aim of this study is to determine the timing of surgical intervention and prognostic factors in patients who present with acute cerebral infarction. Methods : We reviewed retrospectively 23 patients with acute cerebral infarction, who received decompressive craniectomy or conservative treatment from January 2002 to December 2004. We divided patients into two groups according to the treatment modalities [Group 1 : conservative treatment, Group 2 : decompressive craniectomy]. In all patients, the outcome was quantified with Glasgow Outcome Scale and Barthel Index. Results : Of the 23 patients, 11 underwent decompressive craniectomy. With decompressive craniectomy at the time of loss of pupillary light reflex, we were able to prevent death secondary to severe brain edema in all cases. Preoperative Glasgow Coma Scale and loss of pupillary light reflex were significant to the clinical outcome statistically. With conservative treatment, 9 of the 12 patients died secondary to transtentorial herniation. The clinical outcomes of remaining 3 patients were poor. Conclusion : This study confirms the value of life-saving procedure of decompressive craniectomy after acute cerebral infarction. We propose that the loss of pupillary light reflex should be considered one of the most important factors to determine the timing of the decompressive craniectomy.