Moyamoya disease is a very rare condition in which the walls of the internal carotid arteries – which supply blood to important areas of the brain – become thickened. This gradually slows the flow of blood to the brain and increases the likelihood of clot formation, both of which may lead to strokes and transient ischemic attacks. The condition causes small blood vessels to form a network of detours (or “side roads”) to get oxygen to parts of the brain that were formerly served by the narrowed arteries. The way that these tiny blood vessels show up on an angiogram explains the reason for the “moyamoya” name, which means “puff of smoke” in Japanese. Multiple surgical procedures are available for treating this condition, usually with very good results. Without such treatment, patients may experience progressive mental decline along with an increasing risk of life-threatening strokes.