To clarify the epileptogenicity of sevoflurane, electrocorticograms were monitored in seven patients with unruptured cerebral aneurysm under sevoflurane anesthesia. They had no history of epilepsy or other complications. Spike activities on electrocorticography were seen in all seven patients at 3.3% end-tidal sevoflurane. These results suggest that further study is required to evaluate the suitability of sevoflurane for neurosurgical procedures.