To clarify the clinical features of moyamoya disease in Japan, 941 patients with definite moyamoya disease were analyzed from the databases constructed by the Research Committee on Moyamoya Disease, established by the Ministry of Health, Labour and Welfare. Moyamoya disease occurs much more frequently among women than men, with a female-to-male ratio of 1.98. A family history of the disease was observed in 14.9%. The age at onset was characterized by two peaks: one at 5-9 years and another lower peak at around 40 years. Initial clinical features were transient ischemic attack in 46%, infarction in 20%, hemorrhage in 21%, headache in 6%, and epilepsy in 4%. The distribution of the age at onset showed one peak at around 40 years in patients with hemorrhage but two peaks in patients with ischemia.