Recent studies have implicated that seizure occurrence was infrequent in the patients with chronic subdural hematoma(CSDH): However, the exact incidence of seizure occurrence in the patients with CSDH is unclear and the efficacy of prophylactic treatment with anticonvulsants(AC) has not been fully determined. We have analyzed a series of 186 adult patients with CSDH who have been operated at our institute from January 1990 to December 1994 to eneasure the incidence of seizure occurrence and to assess the efficacy of prophylactic treatment with AC. The patients who had past history of seizure disorder or alcoholism were excluded in this study. The results were as follows: 1) Post-operative seizure occurred in only 6 patients(3.2%) and all of the presented seizures were generalized type. 2) Among six patients who presented post-operative seizure, two patients showed additional brain CT findings such as intracerebral hemorrhage or cerebral infarction other than chronic subdural hematoma. 3) Operative method did not show a significant influence on the incidence of seizure occurrence. 4) Prophylactic AC were used in 177 patients and the incidence of seizure occurrence in this group was 3.4% which showed no significant statistical difference with the incidence of seizure occurrence of 0% in 9 patients who were not treated with AC. Above results suggest that prophylactic treatment with AC in the patients with CSDH should be considered cautiously except for the patients with the additional brain parenchymal lesions or past history of seizure disorder.