STUDY DESIGN: Radiological investigation to study the morphologic difference of posterior facet joint in spondylolysis and isthmic spondylolisthesis. OBJECTIVES: To study the correlation of the clinical differences between the one with spondylolysis and another with isthmic spondylolisthesis with morphological analysis of posterior facets of lumbar spine in low back pain, anterior displacement and segmental instability. SUMMARY OF LITERATURE REVEIW: There are many studies for the lumbar facet in back pain, disc degeneration, degenerative spondylolisthesis. However, little is known about the correlation of facet joint between the spondylolysis and isthmic spondylolisthesis. METHODS: This study is done with 27 specimens which contain posterior facet and lamina from 25 patients due to spondylolysis or isthmic spondylolisthesis. We took the computed tomograms in each specimen and obtained the areas and angles of posterior facets of lumbar spine. RESULTS: The group with spondylolysis has mean area(Rt/Lt) of 158.4/159.3mnfand angle(Rt/Lt) of 49.8u/54.0u. The group with isthmic spondylolisthesis has mean area(Rt/Lt) of 172.3/189.6mnfand angle(Rt/Lt) of 44.3u/44.8u. The group with segmental instability has mean(Rt/Lt) area of 155.9/161.8mnfand angle of 48.1u/50.4u. The group without instability has mean area(Rt/Lt) of 173.4/185.2mnfand angle(Rt/Lt) of 46.2u/48.5u. CONCLUSION: There are no significant differences between the morphologic difference of facets with back pain and without back pain. The group with isthmic spondylolisthesis has greater mean area and less mean angle of facet than the group with spondylolysis, but, there are no statistical significant differences(p>0.05). There are no significant morphologic differences of facet between the group with segmental instability and without segmental instability.