In some adult patients with cervical syringomyelia, MRI studies do not identify primary disease within the foramen magnum or spinal canal. To identify the etiology of this idiopathic type of syringomyelia, clinical features and posterior fossa (PF) measurements from 17 of these patients, 17 patients with Chiari I-type syringomyelia, and 32 control subjects were compared. Idiopathic syringomyelia and Chiari I-type syringomyelia manifested central cervical myelopathy and a small PF with narrow CSF spaces, suggesting that they develop by the same mechanism.