Vague Definition of the Diagnostic Guideline of Moyamoya Disease in Japan Moyamoya disease (spontaneous occlusion of the circle of Willis) is discovered in Japan first and most prevalent in the world. Its etiology is still unknown. The diagnostic guideline is proposed by the research committee supported by Japanese Ministry of Health, Labor and Welfare. There is a vague portion in the guideline concerning to the steno-occlusion of the intracranial arteries. (1) At the terminal portion of the internal carotid artery, (2) the proximal portion of the anterior cerebral artery, and (3) the proximal portion of the middle cerebral artery. For fulfill the diagnostic criteria, it is not clearly defined in Japanese language that all the above-mentioned locations (1), (2) and (3) should be involved or at least one of them should be involved. In one report, it is stated as follows: Stenosis or occlusion at the terminal portion of the internal carotid artery and at the proximal portion of the anterior and middle cerebral arteries. but in another report,           Stenosis or occlusion at the terminal portion of the internal carotid artery and/or at the proximal portion of the anterior and/or middle cerebral arteries. In the other reports form the members of the research committee, definition is described as follows:           Stenosis or occlusion is observed at the terminal portion of the intracranial internal carotid artery and/or at the proximal portion of the anterior cerebral artery and/or middle cerebral artery.                or           Stenosis or occlusion of the internal carotid artery at the level of the terminal portion together with abnormalities of the anterior and middle cerebral arteries. These points are important in the diagnosis of moyamoya disease because according to the guideline, it is unclear in some cases that the diagnosis of moyamoya disease is definite or probable. I sent a letter to the editor of No Shinkei Geka (Neurosurgery in Japanese), Japanese Journal in the field of Neurosurgery, in 1999. Unfortunately, there has been no change in the description of the guideline so far. Reference 1. Komiyama M: Diagnostic guideline of the spontaneous occlusion of the circle of Willis. (letter) No Shinkei Geka 27: 481, 1999 (in Japanese)