Moyamoya disease is a rare, progressive cerebrovascular disease caused by blocked arteries at the base of the brain in an area called the basal ganglia. The name "moyamoya" means "cloud of smoke" in Japanese and describes the appearance of the tangle of small vessels formed to compensate for the blockage. Moyamoya disease was first described in Japan and affects individuals all over the world; the incidence is higher in Asian countries than in Europe or North America. The disease mainly affects children, but can also affect adults. In children, the first symptom of Moyamoya disease is often a stroke, or recurrent transient ischemic attacks (TIA, commonly called "mini-strokes"), often accompanied by muscle weakness or paralysis on one side of the body. Adults can also experience these symptoms arising from blocked arteries, but are more likely to experience a hemorrhagic stroke due to bleeding in the brain. Other symptoms can include:

• headache
• to attack
• disturbed consciousness
• involuntary movements
• vision problems
• cognitive and / or sensory disturbances.

Some people with Moyamoya disease have a close relative who is also affected; in these cases, researchers believe that Moyamoya disease is the result of hereditary genetic defects. Examination of the abnormal gene (s) can help reveal the biomechanisms that cause the condition.