(1) The grade of the intensity of brain swelling observed in clinical symptoms and electroencephalogram was the highest in the cases with resection of the cortex and then decreased in the order of those with brain tumors etc., with explorative craniotomy and with lobotomy. The cases with air insufflation by ventricular or lumbar puncture, however, showed almost no changes in electroencephalography. (2) The decrease of the average number of alpha waves per second and the increase of the delta index became most striking in 1-4 postoperative days, then gradually recovered and returned to normal in 15-16 postoperative days, in the epileptics with coritcal resection. (3) The brain swelling was generally marked on the side of craniotomy, and more marked at the part of craniotomy and less at the distant part, with consideration of the frontal domination. (4) The electroencephalographic findings in the brain swelling, such as the changes of the delta index and alpha waves, were parallel with the clinical symptoms. But their appearance as well as their cessation were left a little behind the clinical symptoms. (5) The electroencephalographic changes were not always marked at the part of craniotomy. This is due to the characteristic fact that the frontal lobe reacts more actively i.e. the frontal domination. (6) The water content of the brain was measured in one case of autopsy, and the brain edema was ascertained to be more marked on the side of craniotomy than the other side. (7) The electroencephalographic findings represent the clinical features of the brain
swelling and have enough value for clinical application.