To set up the criteria determining the significant plain radiographic finding suggesting L5/S1 disc space narrowing, a retrospective roentgenographic study was caried out 23 patient having surgically-confirmed L5/S1 disc herniation between 18 and 57 years old. Considering the unique structural and functional characteristics, normal and pathological anatomy of the lumbosacral junction was also reviewed briefly. This paper also re-evaluated and re-lightened the importance to interprete plain lumbosacral roentgenogram more carefully and accurately. For it will certainly serve as the initial screening method to identify the region of concern for the recently thriving Spine Computed Tomography for rapid, non-invasive, effective diagnosis of the universal disaster-lumbar disc herniation. We have obtained the following conclusions; 1. The method to interpret as "positive" L5/S1 disc space narrowing when the central height of L5/S2 disc space is reduded below th central height of L3/4 showed the highest correlation (approximately 70%) with disc herniation, especially in the lower age group below forty (76.9%) 2. The diagnostic significance of L5ulcornerS1 disc space narrowing as a roentgen sign suggesting L5/S1 disc herniation is increasing as the more following items are satisfied; a. When the long-standing symptom and neurologic signs suspecting S1 nerve root compression or irritation are present. b. When the patient is among the lower age group, such as below forty. c. When there is no vertebral lesion causing spinal instability. d. When there are additional findings suggesting L5/S1 disc herniation, especially if they are limited at L5/S1 level. 3. Other conditions having L5/S1 disc space narrowing are the anatomically narrow lumbosacral junction with or without transitional vertebra, the infectious narrowing, the degenerative narrowing without disc herniation, and the isolated disc herniation. Those can be usually differentiated from L5/S1 disc herniation by their different clinical features and plain radiographic findings. findings.