Four cases (2 children and 2 adults) of spontaneous occlusion of the circle of Willis were studied using EEG, somatosensory evoked potential (SEP), cerebral angiography and CT scan. The following results were obtained. 1) Among the above described parameters, SEP was the most useful to detect the ischemic change in the thalamus when the obstruction of the posterior part of the circle of Willis extends, even before a low density lesion of the thalamus appears on CT. 2) In child cases, a decrease in the amplitude N3 of SEP was observed after hyperventilation. It was speculated that this finding of SEP might be correlated with the mild cortical ischemia due to vasoconstriction and "rebuild up" phenomenon in EEG after hyperventilation.