Study Design. A retrospective, cohort study of the kinematics of the lumbar spine of patients with spondylolisthesis compared with asymptomatic normal subjects. Objective. To determine if abnormal motion could be detected in the radiographs of patients with spondylolisthesis. Summary of Background Data. Contrary to the prevailing conviction that lumbar segments affected by lytic spondylolisthesis are unstable, multiple studies have failed to find evidence of increased or abnormal motion at these segments. However, these studies did not use techniques that might reveal abnormalities in the quality of motion, as opposed to its magnitude. Methods. The flexion-extension radiographs of 13 patients with spondylolytic spondylolisthesis were analyzed to determine the location of their instantaneous centers of rotation, and their magnitudes of translation and sagittal rotation. Normative data were obtained by applying the same techniques to the radiographs of 20 asymptomatic subjects. Results. All but 1 of the 13 patients had at least one segment with abnormal motion. Only one patient had excessive translation at the lytic segment. Five patients had minor abnormalities affecting either the lytic segment or ones above, and 6 had paradoxical motion at the lytic segment in which the center of rotation was located above L5, instead of below, and in which L5 translated backward instead of forward during flexion. Conclusions. A proportion of patients with spondylolisthesis had highly abnormal movements but ones with normal magnitudes of motion. Determining the instantaneous axes of rotation reveals the abnormal quality of motion.