Herniation of the spinal cord is a rarely diagnosed condition. After careful study of the literature it seems that all patients present in a similar manner. Clinically there is a disorder of gait compatible with the Brown-Séquard syndrome. On MR there is sudden ventral displacement of the thoracic spinal cord, associated in some cases with an arachnoid cyst. When surgery is effective there is great improvement, so this diagnosis should be remembered in all cases of progressive paraparesis ¹⁾.