OBJECTIVE: To find the developing features of H reflex by the gradual increase of stimulation intensity, to evaluate the variability of each H reflex latency, and to verify the consistency of the side-to-side difference of bilateral H reflex latencies in normal population. METHOD: Fifty normal persons (100 limbs) were studied. H reflexes were elicited using standardized technique from Braddom and Johnson. RESULT: H reflex developed in the stimulation intensity at which M response was not evoked in 46 of total 100 limbs. The latency difference between the longest and shortest in each ipsilateral evoked potential was 0~2.1 ms (mean 0.65+/-0.44 ms). In the side-to-side latency difference between the variously evoked H waves, the mean value of maximal one was 1.28+/-0.63 ms (0.4~2.6 ms), and the mean value of the minimal one was 0.24+/-0.35 ms (0~1.5 ms). The mean side-to-side latency difference between the H waves of maximal amplitudes was 0.69+/-0.46 ms (0.1~1.6 ms), and the mean side-to-side difference between the shortest latencies was 0.61+/-0.53 ms (0~2.1 ms). Among these findings the latency difference of the maximal amplitude was observed in 47 cases with 0~1.5 ms, in 3 cases with more than 1.5 ms. CONCLUSION: In diagnosing the S1 radiculopathy with the side-to-side latency difference it will be reasonable to use the latency of maximal amplitude or the shortest latency as diagnostic tool, and reevaluation will be needed with the diagnostic criteria of 1.5 ms latency difference.