Cauda equina syndrome is an uncommon complication of ankylosing spondylitis (AS) characterized by the slow and insidious development of severe neurologic impairment. Imaging studies usually show a wide lumbar canal with dural ectasia. No medical or surgical treatment has been proven effective. We managed the care of a 66-year-old man who had longstanding AS and clinical features of cauda equina syndrome, including anal incontinence and buttock hypoesthesia. Magnetic resonance imaging demonstrated no cause for these symptoms other than AS. The patient was treated with infliximab, a monoclonal antibody to tumor necrosis factor alpha that is used for the treatment of active AS. After 3 infliximab infusions, sphincter control and sensation were normal. The treatment was continued, and he was still doing well 1 year later. This is the first report of an effective treatment for cauda equina syndrome complicating AS. Our case report strongly supports an inflammatory mechanism to this condition.