The cauda equina syndrome is an uncommon and poorly understood complication of ankylosing spondylitis. The clinical and radiologic findings in five patients with this syndrome are described. Typical findings include cutaneous sensory impairment of the lower limbs and perineum with sphincter disturbances. Motor impairment occurs less frequently, and associated pain is an inconstant feature. Enlargement of the caudal sac and dorsal arachnoid diverticula that erode the lamina and spinous processes are characteristic myelographic and computed tomographic findings. The pathogenesis of the cauda equina syndrome in ankylosing spondylitis remains unknown but may be due to demyelination, post-irradiation ischemia, or compression from spinal arachnoiditis.