Objective : To compare spinopelvic parameters in young adult patients with spondylolysis to those in age-matched patients without spondylolysis and investigate the clinical impact of sagittal spinopelvic parameters in patients with L5 spondylolysis. Methods : From 2009 to 2012, a total of 198 young adult male patients with spondylolysis were identified. Eighty age-matched patients without spondylolysis were also selected. Standing lateral films that included both hip joints were obtained for each subject. Pelvic incidence (PI), sacral slope (SS), pelvic tilt, lumbar lordosis angle, sacral inclination, lumbosacral angle, and sacral table angle were measured in both groups. A comparative study of the spinopelvic parameters of these two groups was performed using SPSS 15.0 (SPSS Inc., Chicago, IL, USA). Results : Among the aforementioned spinopelvic parameters, PI, SS and STA were significantly different between patients with spondylolysis and those without spondylolysis. PI and SS were higher in the spondylolysis group than in the control group, but STA was lower in the spondylolysis group than in the control group. Conclusion : PI and SS were higher in the spondylolysis group than in the control group, but STA was lower in the spondylolysis group than in the control group. Patients with spondylolysis have low STA at birth, which remains constant during growth; a low STA translates into high SS. As a result, PI is also increased in accordance with SS. Therefore, we suggest that STA is an important etiologic factor in young adult patients with L5 spondylolysis.