It has become evident from reports by neurosurgeons, neuroradiologists and pediatricians that the syndromes of transient hemiplegia or convulsions in children (under 16 years of age) and subarachnoid hemorrhage or hemiplegia in adults occur with a particularly high frequency in Japan. Angiographic findings in these patients revealed marked stenosis or obstruction at the terminal portion of bilateral internal carotid artery (most of the cases showed C(1) obstruction) and bilateral arterial vascular networks at the base of the brain. The syndrome was given the name "cerebral basal rete mirabile" by the Japanese and is also variously called moyamoya disease or cerebral arterial rete. Our clinic has experienced thirty-three cases of this disease during the past 12 years (1963~1974). Thirty of these cases (18 adults: 11 males, 7 females and 12 children: 4 males, 8 females) are presented with discussions of the symptoms, clinical course and angiographic findings. Bilateral carotid angiograms were performed in each case. In one particular juvenile case, angiography performed on three different occasions revealed marked changes and these six films are presented for discussion. In sum total, therefore, 36 carotid angiograms from 18 adult cases and 28 from 12 juvenile cases have been discussed. Each angiogram was discussed in terms of grade of narrowing throughout the internal carotid artery, filling of the ophthalmic and posterior communicating arteries, development of the cerebral basal rete mirabile, visualization of the anterior, middle and posterior cerebral arterial regions and development of the leptomeningeal anastomoses. Narrowing was classified into three groups: none, moderate and marked. Filling of the ophthalmic artery and its branches was classified as mild (+), moderate (++) or marked (+++), as was filling in the posterior communicating artery. Development of the cerebral basal rete mirabile and visualization of the anterior, middle and posterior cerebral arterial regions were both reported as good (+++), fair (++) or poor (+). 1) Moderate or marked narrowing throughout the internal carotid arteries was observed in 40% of adult angiograms and 45% of juvenile angiograms. 2) In 9 cases out of 18 adult cases and 9 cases out of 12 juvenile cases, bilateral internal carotid arteries showed almost equal size throughout. 3) The time intervals from the onset of symptoms to angiography examinations showed no statistical correltion to the grade of narrowing of the internal carotid arteries. 4) The collateral circulation to the frontal lobe or frontal base via the medical frontal artery, supra-orbital artery or anterior and posterior ethomoidal artery originating from the ophthalmic artery were detected in 34 out of 64 carotid angiograms (53%). 5) The posterior communicating artery was visible in 53% of adult carotid angiograms and 61% of juvenile carotid angiograms. 6) Leptomeningeal anastomoses were often observed. The most frequently detected leptomeningeal anastomosis originated from the posterior cerebral artery to the middle cerebral artery. The second most frequently detected anastomosis was from the posterior cerebral artery to the anterior cerebral artery. These leptomeningeal anastomoses were detected in the distal regions of the R. splenii, posterior temporal and parietooccipital arteries. Anastomosis from the middle cerebral artery to the posterior cerebral artery was seldom visible. 7) Well developed cerebral basal rete mirabile were more often detected in children, while the poorly developed predominated in adults. 8) 80% of the cases showed almost symmetrical development of cerebral basal rete mirabile. The branching of small vessels forming the cerebral basal rete mirabile or obstruction level of the internal carotid artery was not always symmetrical. This author developed the "visualization index" of internal carotid angiograms. The grade was converted into numbers, namely, - to 0, + to 1, ++ to 2 and +++ to 3. The index was the sum of these numbers depicting the grade of filling of the ophthalmic and posterior communicating arteries, development of cerebral basal rete mirabile, and visualization of anterior, middle and posterior cerebral arteries. 9) The mean visualization index of internal carotid artery of "no narrowing" group was 10.1±1.7 in adult angiograms and 10.4±1.6 in juvenile angiograms. In the moderate narrowing group, 5.9±1.8 in adults and 7.1±1.2 in children and in the marked narrowing group, 2.3±1.2 in adults and 2.5±1.5 in children. These results showed that the narrowing of internal carotid artery closely correlated to the "visualization index" of internal carotid artery. The author reviewed 518 cases of moyamoya disease by personal inquiry to the main neurosurgical, neurological and pediatric clinics in Japan. 10) Sex and age distribution of these 518 cases were as follows. Among these 518 cases, angiographic follow-up was carried out in 68 cases (time interval of follow-up angiography was more than six months). The angiographic changes were classified in three groups, major change, minor change and no change. 11) 8 juvenile cases (male 4 cases, female 4 cases) showed major change. 4 cases became typical moyamoya disease after normal angiograms. 3 cases showed dramatic changed of angiography. Only one case developed typical moyamoya disease with unilateral changes at angiography. 12) 6 juvenile cases (male 3 cases, female 3 cases) and 9 adult cases (male 3 cases, female 6 cases) showed minor changes such as development of collateral pathways via leptomeningeal anastomosis or rete mirabile anastomosis. These minor changes were not typical changes in this disease, because they were often detected in cases with main arterial obstruction of the central nervous system. 13) In short, only 8 cases showed true angiographic changes in 68 cases and these 8 cases were 12% of total cases and 27% of juvenile cases. The author collected 15 autopsy cases from literature in Japan and has personal experience of two others. These 17 cases comprised three juvenile cases (female, 3 cases) and 14 adult case (male, 6 cases, female, 8 cases). Pathological findings of the terminal portion of internal carotid artery and main trunk of anterior and middle cerebral artery were mainly discussed. 14) All cases showed marked thickening of the intima at the terminal portion of the internal carotid artery (C(1)). 13 cases out of 17 cases showed eccentric, stratified thickening of the intima, splitting of the internal elastic lamina and partial thinning of the media. 15) The main trunk of the anterior and middle cerebral arteries (A(1) A(2), M(1) M(2)) showed concentric edematous thickning of the intima, tortuous configuration of the internal elastic lamina and thinning of the media in 13 out of 17 cases. 16) From these pathological findings, it seemed to us that the thickening of the intima at the internal carotid artery and the main trunk of anterior and middle cerebral arteries was of different etiology. 17) The atheromatous changes in the intima were detected in only one juvenile case and 4 adult cases. These cases with atheromatous changes were only one third of the autopsied cases and it was clear that atheromatous change of intima was not the main cause of this disease. 18) Histologically, the abnormal vascular networks showed thin, tortuous walls with irregular dilated lumens and it was very difficult to classify them categorically as venous or arteriolar.