Moyamoya Disease is a Progressive, Occlusive Arteriopathy of the Primitive Internal Carotid Artery Interventional Neuroradiology 9:39-45, 2003 It is extremely difficult to understand the title of this paper even for neurosurgeons. Although scientifically not correct, ontogeny (of the cerebral arterial system) repeats phylogeny in general. In another words, development of the human cerebral arterial system from a conception to form adult arterial system is quite similar to the phyologenetic development from fish, amphibian, reptiles, birds, and then mammals. In the early stage of cerebral arterial development, there are six branchial arches (arteries), the third arch being the origin of the internal carotid artery. I have noticed that the development of this primitive internal carotid artery and its development are quite similar to the disease process of moyamoya disease. Examples are as follows: 1. Even fish has a primitive internal carotid artery and phylogenetic development occurs in the similar way as the phylogenetic development occurs. -> In moyamoya disease, steno-occlusive changes occur initially at the terminal portion of the internal carotid artery. 2. Major portion of the cerebral arteries is formed from the primitive internal carotid artery and vertebro-basilar system has less important contribution. Anterior choroidal artery is the caudal branch of the primitive internal carotid artery and is later transferred (annexed) to the vertebrobasilar system during the course of arterial development. -> In moyamoya disease, no steno-occlusive changes occur initially in the vertebrobasilar system. When it occurs, it is observed at the posterior cerebral artery, which is ontogenetically the part of the primitive internal carotid artery. No change is observed in the basilar artery. 3. Embryologically, the primitive internal carotid artery develops from the carotid bifurcation (at the origin of the posterior communicating artery) centripetally. -> In moyamoya disease, disease process starts at the terminal portion of the internal carotid artery and evolves centripetally. 4. In the early stage of arterial development, the primitive trigeminal and hypoglossal arteries which bridge between the primitive internal carotid artery and the vertebro-basilar system exist temporarily. -> In moyamoya disease, the primitive trigeminal and hypoglossal arteries are frequently discovered. In the discussion of etiology of moyamoya disease, congenital and acquired theories are well known. In consideration of the similarity between the cerebral arterial embryogenesis and disease process of moyamoya disease, genetic factors play at least some roles and the unknown acquired factors may affect the development clinical presentation of moyamoya disease. To the abstract 2004.2.8