The injection of anesthetics and/or steroids into the lumbar facet joints has been advocated in the management of low back pain. During this procedure, arthrography was performed in 70 joints of 27 patients with osteoarthritis and 3 patients with bilateral spondylolysis of a lower lumbar vertebra. The size of the recesses surrounding the facets is emphasized, particularly at the medial aspect of the joint, near the intervertebral foramen. The articular capsule nearly always bursts during or after arthrography, and the path followed by the leaking contrast medium provides an explanation of how injected anesthetics and steroids act in relieving low back pain. Leakage can occur at the lateral aspect of the joint, where the branches of the posterior ramus of the spinal nerve pass, or at the medial side, where the opacification of a joint near a spondylolysis, the contrast medium may spread into the adjacent joint through the defect.