One thousand two hundred patients (600 women and 600 men) aged 18 to 72 years were referred for computed tomographic examination of the lumbosacral spine (L3-S1) after low-back pain or sciatica. Patients with spinal abnormalities other than spina bifida occulta (SBO)-S1 and findings other than posterior herniation of intervertebral disc were not included in this study. All of the patients underwent conventional radiographs of the lumbosacral spine. The incidence of SBO-S1 was higher in younger age groups and decreased with age. Patients with SBO-S1 showed a higher incidence of posterior disc herniation, which increased with age. This can be explained by instability of the base of the lumbar spine caused by SBO-S1, which produces a predisposition to posterior disc herniation. The results were statistically significant.