Decompressive craniotomy in hemispheric infarction has been reported to reduce mortality and improve outcome. Identifying tissue at risk and monitoring the benefit of craniotomy is hardly practical and has not been reported thus far. Perfusion CT was applied before and immediately after decompressive craniotomy in a patient with space-occupying middle cerebral artery (MCA) infarction. Before surgery, perfusion CT revealed malperfused but still vital tissue in the vicinity of the infarction core which returned to normal after decompressive surgery. The final infarct size did not exceed the area of the initial hypodensity on unenhanced CT scan. In critically ill patients, the practicability of perfusion CT allows for demonstration of tissue at risk around the infarct core in space-occupying MCA infarction. Moreover, it may be used to monitor the effect of decompressive craniotomy.