PURPOSE: To evaluate the radiologic findings of intraspinal epidural arachnoid cyst. MATERIALS AND METHODS: Six pateuts with surgically proven intraspinal epidural arachnoid cyst were included in this study. Four were freeof specific past history, but two had a history of trauma. All underwent examination by plain radiography,CT-myelography and MRI, and the following aspects were retrospectively analysed : vertebral pressure erosion,interpedicular distance, enlargement of neural foramina, as seen on plain radiograph, contrast-filling and lateralbulging of lesions through neural foramina on CT-myelograph, and signal intensity, size and shape of margin andepidural fat pattern, as seen on MRI. RESULTS: Three of four congenital intraspinal epidural arachnoid cysts weresingle in the thoracolumbar region, while in the other case, there were multiple cysts in the mid- and lowerthoracic regions. Cysts were equivaleut in size to between four and six vertebral bodies. Plain radiographicfindings of pedicular pressure erosion, widened interpedicular distance, and bilateral neural foraminalenlargement of several contiguous vertebrae were observed in all four cases. One showed posterior vertebralscalloping. On CT-myelograph, a contrast-filled cystic lesion occupying the posterior epidural space, with lateralbulging through neural foramina and anterior displacement of the contrast-filled thecal sac, was seen. On MRI,longitudinally elongated, well-demarcated cysts were seen to be present in the posterior epidural space; theirsignal intensity was the same as in CSF. An epidural fat cap pattern enveloping the upper and lower ends of thecysts was apparent in all cases. In two cases, traumatic intraspinal epidural arachnoid cysts were situated in thethoracolumbar and lumbosacral region, respectively, near a previously injured region and were smaller (equivalentto the height of three vertebral bodies). CT-myelograph and MRI showed that their effect on the thecal sac wascompressive only. CONCLUSION: When pressure erosion of vertebral odies and pedicles of contiguous severalvertebrae in the thoracic or thoracolumbar region is recognized on plain radiograph, intraspinal epiduralarachnoid cyst should be included in differential diagnosis, and for accurate evaluation, CT-myelograph and/or MRIare needed.