OBJECTIVE: To evaluate the values of H-reflex from tibialis anterior in the diagnosis of L5 radiculopathy. METHOD: The subjects were 37 patients and 30 normal controls. The patient group was composed of 28 patients with L5 radiculopathy and 9 patients with S1 radiculopathy, which were confirmed by clinical, radiological, and electrodiagnostic studies. Tibialis anterior H-reflex (TA-H reflex) was recorded from maximally contracting tibialis anterior muscle by averaging technique and submaximal stimulation of common peroneal nerve. Sensitivities and specificities were delineated from the several diagnostic criteria. RESULTS: In the normal controls, mean side to side difference in the TA-H reflex latency was 0.66+/-0.48 msec and mean amplitude ratio was 75+/-16%. The diagnostic criteria of abnormal TA-H reflex were latency difference above 1.62 msec and amplitude ratio less than 42.2%. The abnormal TA-H reflexes were shown in 17 out of 28 patients with L5 radiculopathy and 1 out of 9 patients with S1 radiculopathy. Sensitivity and specificity of TA-H reflex as a diagnostic criteria of L5 radiculopathy were 61% and 89%, respectively. CONCLUSION: Tibialis anterior H-reflex might be useful in the diagnosis of L5 radiculopathy.