A sacral arachnoid cyst was found incidentally in a patient with ankylosing spondylitis. The clinical manifestations were perithecal bony erosions and thecal sac displacement while no associated neurological deficit was detected. This peculiar situation is of special interest as cauda equina syndrome invariably presented in such clinical complex. The pathogenesis of the arachnoid cyst remains unclear. And surgical intervention is only indicated in patients with evidence of nerve root compression.