A 23-year-old woman presented with a 3-month history of increasing weakness and numbness of bilateral legs and dysuria. MRI demonstrated an extensive spinal dorsal cystic lesion from C7 to L2 (figure, A and B). The patient underwent a T9-11 laminectomy, and a cyst-peritoneal shunt was performed using a catheter. Pathology confirmed a diagnosis of arachnoid cyst (figure, C). The cyst reduced in size significantly (figure, D), and the patient is asymptomatic over a 42-month follow-up. Intraspinal arachnoid cysts extending to more than 10 vertebral segments are rare,(1) and cyst-peritoneal shunt for mass effect relief is recommended when complete resection is difficult.(2</SUP)