Object: The diagnosis of tethered cord syndrome (TCS) remains difficult, and the decision to operate is even more complex. The objective of this study was to examine how detailed examination of neurophysiological test results can affect the diagnosis for patients undergoing a surgical cord release. Methods: Patients undergoing tethered spinal cord releases were matched by age and sex with control patients undergoing scoliosis correction in the absence of spinal cord pathology. The latency and width of the P37 peak of the posterior tibial nerve somatosensory evoked potential (SSEP) and the motor evoked potential (MEP) latencies were examined. Immediate changes as a result of the surgical procedure were reported. Results: The width of the P37 response differed significantly between TCS and control patients and changed significantly during the surgical procedure. Nonsignificant trends were seen in SSEP and MEP latencies. Conclusions: The width of the P37 response may be a useful marker for TCS and may play a role in presurgical decision making.