Objective
To compare the diagnostic significance of H reflex and somatosensory evoked potential(SEP) in the diagnosis of S1 radiculopathy, and to explore the possible interpretations on the discrepancies of the test results.

Method
Twenty-eight patients who were diagnosed as a S1 radiculopathy by the clinical and radiological correlations, and thirty-five controls with no evidence of S1 radiculopathy by the clinical or radiological studies were studied by the H reflex, sural SEP, and needle electromyography.

Results
The sensitivity of H reflex study was 64% for the S1 radiculopathy, and the specificity was 86%. The sensitivity and the specificity of the sural SEP study for the S1 radiculopathy were 50% and 80% respectively. Within 1 month from the onset, the sensitivity of H reflex increased to 75%. The needle electromyography showed 19% for the sensitivity and 100% for the specificity. The probability of the S1 radiculopathy was 70% in 10 cases who had the abnormal H reflex and normal SEP values and 36% in 8 cases who had the normal H reflex and abnormal SEP values.

Conclusion
H reflex study was the most sensitive test in the electrodiagnosis of the S1 radiculopathy and the electrodiagnostic significance of sural SEP was obscure.