A previously healthy 12-year-old girl with a 6-month history of progressive pain in her right leg, limping, and atrophy of calf muscles without preceding trauma was referred to our institution for further investigation. On clinical examination, spastic monoparesis of the right leg was noticed with muscle atrophy, increased muscle tone, exaggerated deep tendon reflexes, and a Babinski sign. Sensory examination revealed decreased sensation for touch at the L4 and L5 dermatomes but normal findings for all other qualities. Magnetic resonance imaging of the spine showed kinking of the spinal cord due to right-sided anterolateral herniation of the cord through a defect of the dural sac at the T7 level (Figure). The patient subsequently underwent surgery in which the finding of idiopathic spinal cord herniation was confirmed and reduction was successfully performed. One month after surgery, sensory examination findings were normal and increased muscle tone and exaggerated deep tendon reflexes persisted.