A total of 20 patients with an Arnold-Chiari Type I malformation, i.e. without skeletal abnormality and associated with a syringomyelic syndrome, were studied using the EMI Whole Body Scanner CT5000. Two groups were considered: (i) a retrospective series of 12 patients without intrathecal water soluble contrast media and (ii) a prospective series of eight patients after the introduction of intrathecal Metrizamide. In the first group tonsillar herniation could not be conclusively identified on CT and cord visualisation was possible in only two cases. In the second group, tonsillar herniation and cord expansion were consistently demonstrated. Cord cavitation has also been detected. In both groups it was possible to assess the size and position of the ventricular system. The results of this study indicate that evaluation of the upper cervical cord cerebellar tonsillar herniation, may be achieved, but only following opacification of the CSF. The precise extent of cerebellar tonsillar herniation may be difficult to evaluate. The reasons for these findings are discussed.