1. A case of low back pain due to a sacral extradural cyst is reported. Radiographs of the sacrum showed an ovoid expansion of the sacral canal. Myelography and exploratory laminectomy revealed an extradural cyst associated with a defect in the dural root sleeve surrounding the second left sacral roots. Relief of symptoms followed evacuation of the cyst and repair of the defect.

2. The differential diagnosis of the condition, the varieties of extradural cyst, and the features which distinguish them from perineurial cysts are described.