The presence of cysts within the sacral spinal canal, so-called sacral cysts, is described in literature. These include 'sacral perineural cyst', 'sacral extradural cyst', 'occult intrasacral meningocele' and 'anterior sacral meningocele'. Sacral perineural cyst in these cystic disorders was first described as an incidental autopsy finding by Tarlov in 1938. Since then, several reports have been made describing the sign and symptom, neurological findings, roentgenographic diagnosis and cause and origin of the sacral perineural cysts, although many problems are not yet solved satisfactorily. This cyst occurs on the extradural components of sacral or coccygeal nerve roots. Although most are asymptomatic, these occasionally cause low back pain, sciatic and sacrococcygeal pain, sensory and motor disturbance in the lower extremities, and urinary dysfunction, which symptoms are similar to those brought on by lumbar disc herniation. In 1948, Tarlov reported a case of sciatic pain due to a perineural cyst, the removal of which relieved the symptoms. Symptoms occur because adjacent nerve roots are impinged upon by the thin-walled, fluid-filled cysts, which are formed in a space between the endoneurium and the perineurium. Microscopically, the cyst walls consist of peripheral nerve fibers or ganglionic cells covered with meningeal epithelium. Communication of the cyst with subarachnoid cerebrospinal fluid may be poor, but myelogram and CT myelogram demonstrate the cysts filling with contrast media. With the advent of magnetic resonance imaging (MRI), imaging of the sacral perineural cysts has improved. Recently we had the opportunity to evaluate a patient in whom perineural cysts had caused considerable erosion of the sacrum.(ABSTRACT TRUNCATED AT 250 WORDS)