A 25 year old patient who had low back pain and sciatica was diagnosed as having a lumbar intraspinal extradural synovial cyst adjacent to a facet joint between the third and fourth lumbar vertebrae. The extradural defects defined with positive contrast myelography and postmyelographic computed tomographic scanning were posterolateral to the dural sac. Biopsy revealed the findings compatible with synovial cyst. Treatment consisted of laminectomy and surgical excision of the cyst.