Cauda equina syndrome is a rare neurologic complication in patient with long-standing ankylosing spondylitis. Authors report a case of cauda equina syndrome. in ankylosing spondylitis. On plain radiographs, typical “bamboo spine” and fusion of sacroiliac joints were noted. MRI of the lumbar spine revealed arachnoid diverticulae which were isointense to cerebrospinal fluid on all spin echo sequences, and showed bony erosion and scalloping of posterior arches.