STUDY DESIGN: A retrospective study was conducted on the preoperative neurologic symptoms of patients with lumbar herniated discs. OBJECTIVE: To evaluate the possibility that the muscle innervation pattern and the sensory dermatomes of lumbar nerve roots are altered when a lumbosacral transitional vertebra is present. SUMMARY OF BACKGROUND DATA: In 1962, McCulloch et al suggested with intraoperative recordings that the innervation pattern of the lumbar nerve roots may be altered when a lumbosacral transitional vertebra is present. However, this result was not repeated in the study by Young et al in 1983. METHODS: We retrospectively reviewed the charts of 62 consecutive patients who underwent microdiscectomy for lumbar herniated discs. Lumbarized S1 vertebra was present in 10 of 62 patients (16%). Among these 10 patients, 8 had herniated discs at L5-S1 compressing the S1 nerve root. In the 52 normally configured patients, 22 had herniated discs at L5-S1 compressing the S1 nerve root, and 15 had herniated discs at L4-L5 compressing the L5 nerve root. The preoperative neurologic symptoms caused by the S1 nerve root compression in the patients with lumbarized S1 vertebrae were compared with the symptoms caused by either L5 or S1 nerve root compression in the patients with normal configuration. RESULTS: The distribution of motor weakness caused by the S1 nerve root compression was significantly different between the patients with lumbarized S1 and those with the normal configuration. The motor weakness caused by the S1 root compression in the patients with lumbarized S1 was similar to that of the L5 nerve root compression in the normal configuration. Analysis of sensory symptoms showed similar results. CONCLUSIONS: Our findings suggest that the function of lumbosacral nerve roots is altered in patients with lumbarized S1 so that the S1 nerve root serves the usual function of the L5 nerve root.