Intradural lumbar disc herniation is a rare pathological entity. The pathogenesis of intradural lumbar disc herniation is not known clearly. Intradural disc herniations usually occurred at the L4-L5 levels but have also been reported at other levels. However, intradural disc herniation at L5-S1 is quite rare. There are approximately nine reports in the English literature of intraradicular disc herniation at L5-S1. We described a 61-yr-old man with suspected intradural mass at the level of L5-S1 space. The patient presented with pain in the lower back and both lower legs for 4 months and a sudden exacerbation of the symptoms for 3 days. Gadolinium-enhanced magnetic resonance imaging (MRI) demonstrated a large disc herniation at the L5-S1 level with an intradural component. L5 and S1 laminectomy was performed, and dura was swollen and immobile. Subsequent durotomy was performed and an intradural disc fragment was removed. The patient had full recovery in 3 months. Intradural lumbar disc herniation must be considered in the differential diagnosis of mass lesions in the spinal canal. Contrast-enhanced MRI scans are useful to differentiate a herniated disc from a disc space infection or tumor.