Facet arthrograms in patients with lumbar spondylolyses show spread of contrast medium through pars interarticularis defects into the adjacent ipsilateral facet joint space and across the midline into the contralateral joint space. Transverse and sagittal sections of facet joints from 141 postmortem lumbar spines were studied. Five examples of spondylolysis were found. Anatomic studies of the relationship of the joint recesses to the pars interarticularis demonstrated the basis for the patterns of spread of contrast medium observed radiographically in this and previous series. The pars interarticularis forms the only boundary between the inferior recess of one facet joint and the superior recess of the adjacent joint. Fracture through the pars interarticularis establishes communication between adjacent ipsilateral joints and opens a communication to the retrodural space, through which contrast medium can track to the contralateral joint.