An unverified disease called "Moyamoya Disease" or "Spontaneous occlusion of the circle of Willis" has been recently reported as a disease entity by some Japanese researchers. Since the first report of this disease by Shimizu and the author in 1955, many cases have been reported not only in Japan but in many countries outside Japan. It has been already clarified either clinically or pathologically, that, in the Moyamoya Disease, the most important finding is the basal arterial occlusive change of unknown etiology and the Moyamoya Phenomenon is only nonspecific neuroradiological change as the extraordinary dilated collaterals via the striate arteries, perforators etc. However, the real cause of the arterial obstruction is still obscure in the so-called "true Moyamoya Disease". Further studies will be necessary in order to establish a new clinical entity related to the Moyamoya Disease. However, under existing situations, the Moyamoya Disease must be strictly differentiated from the Moyamoya Phenomenon which can be frequently observed among cases with basal occlusion of known and unknown origin.