The joint spaces and synovium of the lumbar facet joints extend beyond the articular surfaces of the joint in the majority of adults. The authors correlated magnetic resonance (MR) imaging, computed tomographic (CT), and axial cryomicrotome sections of 66 facet joints in nine cadavers. Extensions of the synovium and joint space along the superior and inferior articular processes, under the ligamentum flavum, and into the ligamentum flavum could be recognized with use of MR and, less successfully, CT. The injection of a paramagnetic contrast medium into the facet joint facilitated visualization of the capsule on MR images. On the ventral aspect of the lumbar facet joint, MR images showed regions of high signal intensity where the joint space extended into the ligamentum flavum or between the ligamentum flavum and lamina. On the dorsal aspect of the joint, MR demonstrated prominence of the fibrous joint capsule where the joint space extended under it along the inferior articular process or along the superior articular process. The variable appearance of the ventral and dorsal aspects of the lumbar facet joint on CT and MR images is due to extension of the synovium and joint space.