A 56-year-old Japanese woman underwent hemi-arch replacement with selective cerebral perfusion for acute aortic dissection of DeBakey type I. A postoperative computed tomographic scan of the brain showed extensive infarction of the right hemisphere, a midline shift, and right uncal herniation. The patient received right fronto-temporo-parietal craniectomy and external decompression followed by mild brain hypothermia, targeting a rectal temperature of 34°C. Despite residual left hemiparesis, she became alert and successfully recovered. She was discharged 147 days after the operation. We conclude that external decompression with mild brain hypothermia was therapeutically useful, with no major complications, for the treatment of extensive cerebral infarction associated with acute aortic dissection.