A middle-aged man with a history of lower back pain and radiculopathy was suddenly taken ill with a new onset lower back pain, lower limb weakness, sensory changes in the saddle and perineal areas and urinary retention. The patient was admitted to hospital within a few hours of the onset of the symptoms. The cauda equina syndrome was diagnosed. A CT scan, performed before the onset of the cauda equina syndrome, revealed a massive herniation of the L4-L5 disk with dural sac compression and narrowing of the canal. Urgent surgery was undertaken and the L4-L5 disk sequester was removed. Sensory changes and lower limb weakness passed in a few days after the surgery. The bladder function gradually improved during the subsequent weeks.