Magnetic resonance (MR) imaging and computed tomography (CT) were used to study severe syringomyelia, in which the syrinx was so eccentric relative to the center of the spinal cord that at initial examination it appeared to be an exophytic extramedullary mass, in five of six patients with type 2 Arnold-Chiari malformation and one patient with an intramedullary tumor. Sagittal and axial images were routinely obtained at 1.5 T; CT was performed after intrathecal injection of contrast material. On sagittal MR images, the exophytic component of the syrinx typically displaced the spinal cord peripherally and mimicked an extramedullary intradural lesion such as an arachnoid cyst or meningioma. On axial MR images, the gradual appearance and disappearance of this component could be traced as it compressed the spinal cord, which had a signet-ring appearance at the equator of the syrinx. It is concluded that both MR imaging and postmyelographic CT can reveal the characteristics of this lesion on axial images, but MR imaging is superior because it enables direct sagittal imaging.