Unilateral and Quasi-Moyamoya disease The diagnostic criteria of moyamoya disease are summarized as follows: 1. Steno-occlusive changes at the terminal portions of the intracranial internal carotid arteries and development of "moyamoya vessels" at the base of the brain. 2. The above-mentioned findings are observed bilaterally. When the lesion is bilateral, the diagnosis of moyamoya disease is definite, but otherwise it is probable . Exclusion criteria include arteriosclerosis, auto-immune diseases, brain tumor, head injury, post-irradiation, von Recklinghausen's disease, Down's syndrome, etc. Except for definite moyamoya disease, similar clinical conditions are not well understood. (1) Typical angiographic findings solely on one side are observed, but the opposite side is completely normal. (2) Typical angiographic findings are observed bilaterally, but the above-mentioned systemic disease is associated with. Another condition (1) + (2), that is, unilateral lesion in association with the systemic disease exists.       (1) is called "unilateral moyamoya disease", and (2) is called "quasi- or akin-moyamoya disease ". Among unilateral moyamoya diseases, some remain as unilateral one, but some become bilateral diseases. Because etiologies of unilateral and quasi-moyamoya diseases are not well elucidated, they are usually treated in the similar fashion as does moyamoya disease.