Lumbar spinal extradural arachnoid cysts are rare, expanding lesions and they commonly occur in the thoracic spine and lie posterolaterally. Their pathogenesis is still unclear and it is claimed that arachnoid cysts are the result of a widening of the septum posticum. The mechanism of cyst enlargement may be active CSF secretion from residual arachnoid matter or ball-valve effect. Congenital, hereditary, idiopathic, and inflammatory etiologies have been proposed. We report an extradural lumbar arachnoid cyst associated with a lumbar disc herniation. We only performed L1-2 discectomy and left L1-L2 foraminotomy to evaluate the role of disc herniation and spinal canal stenosis on the development of ball-valve effect which made the cyst enlarge.