Three cases of post-traumatic syringomyelia are presented and the mechanism of syrinx formation is discussed. Two cases were examined radiologically. Computed tomography and magnetic resonance images (MRI) showed an expansive syrinx with adhesive arachnoiditis in the thoracic levels below the injury site and a localized syrinx on the posterolateral gray matter in the cervical levels above the injury site. These syrinxes existed below the C2 level and had no communication with the fourth ventricle. The other was an autopsy case. Postmortem examination revealed that a syrinx existed from C2 to Th6 and had no communication with the fourth ventricle or the central canal. It is concluded that small traumatic cavities in the gray matter evolve to an extensive syrinx by cerebrospinal fluid (CSF) entering via the posterior root entry zone, and adhesive arachnoiditis is an important factor in increasing the CSF which is entering. MRI was useful for the diagnosis.