OBJECTIVE: To analyze morphologic differences between only spondylolysis group and mild spondylolisthesis one in young-aged persons and to find useful parameters for radiological assessment of mild spondylolisthesis. METHOD: Twenty patients with only lumbar spondylolysis, 18 patients with mild lumbar spondylolisthesis and 19 normal subjects were recruited in this study. Their radiological findings were examined. The films of subjects were evaluated with respect to variables describing wedging of the spondylolytic vertebra, relative thickness and lengths of the transverse processes. The evaluation was made with attention to possible signs which could mean vertebral slipping. The lumbar index reflects the degree of wedge deformity of the spondylolytic vertebra. RESULTS: Lumbar index was significantly lower in spondylolisthesis group than only spondylolysis group. There was no significant difference in relative thickness of L5 transverse process between two groups. The incidence of a midline lumbar or sacral defect in the spondylolisthesis group was higher than other groups. CONCLUSION: Our results support the usefulness of lumbar index as a supplement parameter for radiological assessment of mild spondylolisthesis.