Moyamoya disease is a rare condition in which the walls of the carotid arteries thicken and the arteries narrow, restricting or blocking the blood flow to the brain. Moyamoya disease is progressive, which means that the vessels will continue to narrow over time, and affects mostly children and adolescents. The cause of moyamoya disease is unknown, although genetics are a likely factor. In response to the narrowing of the blood vessels, the body will begin to grow many small blood vessels to bypass the blockage. “Moyamoya” means “puff of smoke” in Japanese, and refers to the appearance of these networks of new blood vessels on a cerebral angiogram (a test that shows the blood flow in the brain). The condition first was identified in Japan, where its incidence is higher.