OBJECTIVE: Hemifacial spasm has characteristic and specific electrophysiological finding, lateral spread response(LSR). We study the correlation between change of lateral spread response during microvascular decompression(MVD) and clinical outcome after MVD. METHODS: Sixty two patients with hemifacial spasm who were treated with microvascular decompression from March 2000 to February 2003 were included in this study. The monitoring of intraoperative facial electromyography(EMG) and brain stem auditory evoked potential were performed. RESULTS: In 28 (44.7%) patients, there was persistence of lateral spread response after vascular decompression in root exit zone of facial nerve. Among these 28 patients, 9 had mild hemifacial spasm at discharge. Three out of 34 patients who had intraoperative disappearance of lateral spread response after MVD had mild hemifacial spasm. But Both groups, disappearance of LSR (Group I), and persistence (Group II) had only 2 patients with mild hemifacial spasm, and 5 patients at 3 months, respectively. CONCLUSION: Although intraoperative EMG monitoring is very useful in assessing the efficacy of MVD, the clinical outcome of MVD in patient with hemifacial spasm does not always correlate with EMG finding. The prognostic value of intraoperative LSR monitoring in the long-term results is questionable.