Acute ischemic stroke involving the entire vascular distribution of a carotid or middle cerebral artery can cause massive cerebral edema. This study evaluated external decompression for the treatment of massive stroke and analyzed possible prognostic factors. Twenty-four patients with acute massive cerebral infarction, which had progressed to tentorial herniation and impending death, underwent external decompression after medical therapy failed to achieve an effective response. The neurological outcome 2 months after surgery using the Glasgow Outcome Scale was severe disability in 14 patients, vegetative state in two, and death in eight. The overall mortality was 33%. Various characteristics (age, sex, etiology, side of hemispheric infarction, pupillary asymmetry, Japan Coma Scale, distribution of infarction, hemorrhagic infarction, midline shift, tentorial herniation) were evaluated to determine the factors associated with high mortality after surgical intervention. There was no statistically significant relationship between any variable and mortality. Mortality was especially high in the patients with preoperative consciousness level of 200, anterior, middle, and posterior cerebral artery territory infarction, and stage III of tentorial herniation. Postoperatively, all patients with severe disability returned to a clear level of consciousness. Six patients with dominant hemisphere stroke had some measure of communicative skills in spite of aphasia. External decompression is a life-saving treatment for patients with massive cerebral infarction and can provide a reasonable quality of life even for those with dominant hemisphere strokes. Decompressive surgery should be considered and performed as soon as possible if computed tomography demonstrates signs of descending tentorial herniation.