Previous studies have reported increased cerebral blood flow (CBF) velocity after decompressive craniectomy in traumatic brain injury (TBI) patients. A 27-year-old man presented with clinical and tomographic signs of cerebral herniation secondary to TBI. Prior to decompressive craniectomy, hemodynamic study by perfusion computed tomography (CT) indicated diffuse cerebral hyperperfusion. Following surgical decompression, the patient recovered neurologically and perfusion CT disclosed a decrease in the intensity of cerebral perfusion. The patient's blood pressure levels were similar at both pre- and postoperative perfusion CT examinations. This finding provides indirect evidence that decompressive craniectomy may improve mechanisms of CBF regulation in TBI, providing pathophysiological insights in the cerebral hemodynamics of TBI patients. This is the first report analyzing the hemodynamic changes through perfusion CT (PCT) in a patient with decompressive craniotomy due to TBI.