In 12 patients with myelographic evidence of bilateral root involvement at the L3-L4 or L4-L5 levels postmyelographic computerized tomography (CT) studies were performed in flexion and extension. They showed concentric narrowing of the spinal canal in extension and widening with relief of nerve root involvement in flexion. This could be attributed to the presence of marked degenerative hypertrophy of the facet joints, narrowing the available space for dural sac and emerging root sleeves. In extension of the lumbar spine, bulging of the disc toward the hypertrophic facets causes a pincers mechanism at the anterolateral angles of the spinal canal with the risk of bilateral root compression. This mechanism is enhanced in these cases by marked dorsal indentation of the dural sac because of anterior movement of the dorsal fat pad in extension. The authors believe that the radiologically described mechanism forms the anatomic basis of neurogenic claudication and posture-dependent sciatica.