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Objectives: To determine the diagnostic value of area of H reflex in patients diagnosed with S1 radiculopathy. Methods: We reviewed medical records of subjects who underwent EMG study including H reflex between January 2004 and May 2010. Of a total of 2612 subjects, only patients with S1 root involvement confirmed during surgery were recruited. At the end, 47 patients were analyzed. Results: Among the 47 patients, 28 patients presented no response on the affected side and 7 showed no response bilaterally. Ten subjects with bilateral evoked potential were analyzed to calculate H latency, H amplitude, H area and H to M area ratio. Two did not show response on the unaffected side. In 10 patients with bilateral evoked potential, only H area showed significant difference between affected and unaffected sides. Using the H area as a parameter, 37 patients (28 - no response, 9 - H area smaller than that of the unaffected side) or 35 patients (28 - no response, 7 - H area less than 50% of the unaffected side) met the criteria. Sensitivities were 78.7% and 74.5%, respectively. Conclusion: We concluded that area of H reflex may increase the sensitivity in diagnosis of S1 radiculopathy.