The changes in the three-dimensional motion behavior of the lumbar motion segments, as affected by partial discectomy, are presented. The injured specimens, when subjected to flexion, extension, lateral bending, or axial torsional loads, showed significant increases in their major motions when compared with the corresponding intact specimen. No significant increases in the coupled motions were observed. These results suggest that it is better to excise as little of the nucleus as possible at surgery, since it may limit the increase in motion due to the injury.