Seventy-five patients with a clinical diagnosis of syringomyelia were examined by computed tomography after intrathecal injection of metrizamide. A central cavity was demonstrated in 67 patients. Tilting the patient head down did not increase the rate of cavity opacification. This evidence favors transneural migration of metrizamide into the cavity. The spinal cord was measurably enlarged in only a minority of the patients. In some, the cavity appeared to have clefts or wall defects. These results are discussed according to the etiopathogenic theories advanced by Gardner, Aboulker, and Williams.