A 20-year-old woman with selective cauda equina hypertrophy presented with muscle weakness and severe pain in the lower extremities. Serial immunotherapy was not effective. We performed biopsy of the cauda equina, and laminectomy and duraplasty of the thoracolumbar region. Biopsy revealed marked infiltration of small lymphocytes and foamy macrophages in the endoneurium. Three years after decompression surgery, her symptoms have improved slightly without progression or relapse. This is the first case of selective cauda equina hypertrophy with idiopathic inflammation. We propose that this is a new disease entity.