Case report of patient with low back pain irradiating to the left lower limb, with anesthesia in the ipsilateral gluteal region. Operation confirmed protrusion of the disk in the 4th intervertebral space and showed a cyst in each of left dorsal roots S2, S3 and S4; dorsal root St of the same side was strongly thickened. The various etiopathogenic concepts of perineurial cysts are discussed.