PURPOSE: To evaluate the finding useful for differential diagnosis and associated abnormalities of isthmic spondylolisthesis and degenerative spondylolisthesis on CT. MATERIALS AND METHODS: We reviewed retrospectively the CT images of 164 patients who were diagnosed spondylolisthesis. One hundred twelve patients had isthmic spondylolisthesis 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 horizontal plane, an irregular surface, a sclerotic margin, and protruding hypertrophic bony spur in the spinal canal. The most frequently associated structural abnormality was a herniated nucleus pulposus at the upper level of the defect. Degenerative spondylolisthesis most frequently occurred at L4-5 and were grade I. The degenerative facet joint 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 the level of the displacement. CONCLUSION: In spondylolisthesis, the findings in CT were valuable for differential diagnosis of isthmic and degenerative types and the detection of associated symptomatic abnormalities.