Purpose: To reveal the primary epileptogenic focus by analyzing peak latency between interhemispheric sharp waves in Lennox-Gastaut Syndrome (LGS) with primary and/or secondary generalized epileptiform discharges. Method: We retrospectively analysed preoperative electroencephalographic data of 26 patients with intractable LGS and who had surgical treatments including cortical resection or complete corpus callosotomy at Severance Children’s Hospital from August 2004 to December 2008. Patients were divided into 15 patients of group I and 12 patients of group II depending whether epileptogenic focus could be lateralized or not after epilepsy surgery. Group I obtained Engel’s class I outcome by ipsilateral cortical resection and 2ndary generalized epileptiform discharges disappeared. In group II, bilateral independent epileptiform discharges had revealed after complete corpus callosotomy. In distinct generalized epileptiform discharges, interhemispheric sharp waves with the highest negative potential on average montage were selected and peak latencies between them were manually measured by increasing the recording speed into 120 mm per second. Results: Peak latency between interhemispheric sharp waves showed significant difference between two groups. Peak lantency during an awake state in group I was calculated as amount of 0.0163 seconds, and in group II as 0.0041 seconds (p-value = 0.0005). Peak latency during an asleep state in group I calculated as amount of 0.0174 seconds, and in group II, as 0.0044 seconds (p-value = 0.0005). Pathologic findings in group I were as followings; microdysgenesis in 3 patients, cortical dysplasia in 4 patients, gliosis in 4 patients and in 4 patients any pathologic findings could not be detected despite of Engel’s class I outcome after cortical resection. Conclusion: Digital electroencephalographic analysis for measuring the peak latency between interhemispheric sharp waves in LGS could provide useful information to lateralize and determine primary epileptogenic region in LGS.