Fourty and eight cases of epileptjcs were studied electroencephalographjcally by Aird's method before and after the operation and their foci were determined. The foci were compared with those of other methods and they were investigated before and after the operation. The results were as follows : (1) The focus of idiopathic epilepsy in this method was mostly in temporal region, and reached 72% of all cases (total of both sides). (2) The elctroencephalographic focus was on the same side of the focus determined by provocation of the convulsion to 66.6% in idiopathic epilepsy and 88.8% in symptomatic epilepsy. (3) The side of electroencephalographic focus was on the same side of the focus determined by adversive movement to 72.7% in idiopathic epilepsy and 57.1% in symptomatic epilepsy. (4) The side of the electroencephalographic focus was on the same side of the focus determined by pneumoencephalography to 92.6% in idiopathic epilepsy and to 100% in symptomatic epilepsy. (5) The side of the electroenccphalographic focus was on the same side of the determined focus to 87.8% in idiopathic epilepsy to 100% in symptomatic epilepsy and totally 92.9%, (6) The side of the presumed focus by each examination was on the same side of the synthetically determined focus in the order of eleclroencephalography (92.9%), pneumoencephalography (86.5%), initial side of provocated convulsion (72.2%), adversive movement (57.1%). The order was evaluated to be similar in idiopathic as well as symptomatic epilepsy, but generally the grade of their coincidency was higher than symptomatic epilepsy. (7) The epileptics, who showed a displacement, of focus to the frontal lobe after operation, took a good course and the convulsion decreased or dispersed. On the contrary, those, in whom no traveling of the focus or the into the motor area of the same side was seen, usually took an unfavourable course of the convulsive seizure. (8) Most of the symptomatic epilepsy showed no focus in the electroencephalogram after the operation, and accordingly no more convulsion. (9) This method is able to be a good indicator for determination of epileptic focus and judgement of the effect of therapy.