Cauda equina syndrome is a rare complication in the late stage of ankylosing spondylitis, for which approximately 60 cases have been reported in the literature. The cause of the syndrome is unclear, and there is no effective treatment. Recently lumboperitoneal shunt was reported to have been effective in two patients. In our study, we performed lumboperitoneal shunt in a patient and evaluated the condition after the operation compared with that preoperatively. Some alleviation of neurologic symptoms was observed for 6 months after operation. Histopathologic examination of the dural diverticulum revealed a residual change after old inflammation. Lumboperitoneal shunt was an effective surgical treatment for cauda equina syndrome in this patient with ankylosing spondylitis, but its effects were not extreme. Arachnoiditis is suggested to be involved in the pathogenesis of cauda equina syndrome.