Patients with massive brain swelling undergo external decompressive craniectomy to manage intracranial pressure. Following supratentorial craniectomy, and after the brain swelling is relieved, cranioplasty is performed. Although feasibility of the surgery is usually assessed by CT scanning in a supine position, it is sometimes difficult to determine whether the surgery can be performed safely. Although nine patients underwent a decompressive craniectomy during the study period, only six patients could undergo brain CT-first in a supine position and next in a lateral decubitus position with the surgical side upward-before cranioplasty. On CT images, the distance from the midline to the brain surface was measured on the image where brain bulging was maximal, and the bulging was calculated by comparing the image with the distance measured on the contralateral side. In all cases, brain bulging decreased with this change in position. The decrease ranged from 5.5-9.2mm(mean 7.1mm). Patients with brain bulging of 2.8-3.6mm in the lateral decubitus position needed no additional procedure, or only required drainage of a very small amount of cerebrospinal fluid(CSF)from the brain surface. Those with brain bulging of 5.1-12mm showed ventricular dilatation on CT images, and required ventricular puncture or spinal CSF drainage to decrease brain bulging for cranioplasty. We believe that the lateral decubitus position, with the surgical side upward, ameliorates the local brain shift induced by gravity. A lateral position during CT simulates the surgical head position for cranioplasty and can help to assess whether cranioplasty is feasible.