The authors described twins of ten-year-old girls who developed Moyamoya disease at the same time. The family history showed no abnormality. They were born in 1973 under the cesarean section for the early rupture of the membranes and weak pains. The first case experienced tonic convulsions, and abnormal findings on the EEG were pointed out on May, 1979. Seven months later, CT scan was performed in another hospital, which showed the findings of brain atrophy. She experienced absence attacks on January, 1980, and had the first admission. She was diagnosed as epilepsy. Right carotid angiography (CAG) was performed, which showed no abnormal findings. On August, 1983, she complained transient ischemic attacks (TIA) of right hemiweakness and sometimes loss of consciousness for 5 to 10 minutes. She had the second admission on September, 1984. Growing status and neuro-psychological findings were normal. The EEG seemed almost normal, but she complained a TIA on right upper extremity two minutes after the hyperventilation, so the examination was stopped. CT findings was the same as that performed in 1979. Bilateral CAG showed marked stenosis of the internal carotid arteries (ICA) at the siphon and basal moyamoya vessels. Vertebral angiography had no sterno -occlusive lesions. Left encephalo-duro-arterio-synangiosis (EDAS) was performed, and six weeks later, right EDAS. Postoperatively, TIAs disappeared and she had a good clinical course. The second case had a similar history as described in case 1, except the absence of epileptic attacks. EEG abnormality was pointed out, and she also had the diagnosis of epilepsy.(ABSTRACT TRUNCATED AT 250 WORDS)