The authors described the case of a 39-year-old man with Klippel-Trénaunay syndrome, who had an extradural meningeal cyst expanding into intervertebral foramen of lumbar 2 and 3 vertebrae. The patient suffered from low back pain radiating to the left lower extremity. Magnetic resonance imaging revealed a huge extradural meningeal cyst growing through intervertebral foramen far laterally. Awidened neural foramen of L2 and L3 vertebrae was observed on his plain radiography. The cyst was totally excised after tying the ostium connecting the subarachnoid space of the dural sac. This case supports the congenital theory in the pathogenesis of spinal cysts because the Klippel-Trénaunay syndrome is a congenital disorder including a mesodermal abnormality which may be the causative factor for a congenital defect in dura.