Moyamoya is the name given to an angiographic picture consisting of bilateral intracranial carotid artery occlusion associated with telangiectatic vessels in the region of the basal ganglia. It appears that patients with this angiographic finding can be divided into two distinct clinical groups. The first consists of children with a "primary" form of moyamoya that causes alternating hemiplegia, frequently of nonprogressive nature. The second consists of children and adults with various underlying diseases who develop an "acquired," usually progressive form of moyamoya. The striking predominance of Japanese patients with primary moyamoya and its high familial occurrence suggests that the primary form represents a hereditary malformation of the cerebral vascular bed. We report three additional cases of moyamoya. One case represents the primary form and the two are examples of the acquired form in a child and an adult.