Although cranioplasty seems to be a simple procedure, fatal complication of development of diffuse severe cerebral edema following cranioplasty although unusual have been reported in a few cases. The mechanism for this occurrence is still speculative. A 38-year-old male patient presented with a history of having undergone left decompressive hemicraniectomy following a road traffic accident with traumatic left intracranial internal carotid artery dissection leading to anterior cerebral artery and middle cerebral artery territory infarct and endovascular embolization of traumatic type 1 left caroticocavernous fistula. The preoperative computed tomography scan of the brain revealed left frontotemperoparietal craniectomy defect with sunken flap and diffuse encephalomalacia with gliosis of the entire left cerebral hemisphere. Immediately following an uneventful cranioplasty surgery with titanium mesh, the patient developed severe hypotension and dilated fixed pupils. Postoperative imaging revealed diffuse severe cerebral edema in bilateral hemispheres with a significant midline shift toward the ipsilateral (left) side, that is, toward the side of cranioplasty. The patient immediately underwent removal of the titanium mesh, and despite all efforts, the patient had a fatal outcome on postoperative day 5. Although this type of fatal complication of diffuse severe cerebral edema is rare in postcranioplasty patients, neurosurgeons must be aware of this complication and close monitoring postprocedure is important, especially in patients with a large craniectomy defect and sunken skin flap.