Purpose : To evaluate the finding useful for differential diagnosis and associated abnormalities of isthmicspondylolisthesis and degenerative spondylolisthesis on CT. Materials and Methods : We reviewed retrospectivelythe CT images of 164 patients who were diagnosed spondylolisthesis. One hundred twelve patients had isthmicspondylolisthesis and 52 patients had degenerative spondylolisthesis. Results : Isthmic spondylolisthesis mostfrequently occurred at L5. The degree of anterior displacement was grade I and II. The defect had a horizontalplane, an irregular surface, a sclerotic margin, and protruding hypertrophic bony spur in the spinal canal. Themost frequently associated structural abnormality was a herniated nucleus pulposus at the upper level of thedefect. Degenerative spondylolisthesis most frequently occurred at L4-5 and were grade I. The degenerative facetjoint had a vertical plane, a hypertrophic bony spur, and a vacuum facet phenomenon. We frequently detected apseudobulging disk. The most frequently associated structural abnormality was a herniated nucleus pulposus at thelevel of the displacement. Conclusion : In spondylolisthesis, the findings in CT were valuable for differentialdiagnosis of isthmic and degenerative types and the detection of associated symptomatic abnormalities.