Cerebrovascular accidents due to Moyamoya disease, a disorder characterized by arterial stenosis at the base of the brain accompanied by typical net-like collateral vessels, occurred in two young Japanese women with Graves' disease when they were in thyrotoxicosis. In one patient, a second attack of cerebral infarction occurred with the recurrence of thyrotoxicosis. Association of Moyamoya disease and Graves' thyrotoxicosis is rare and the pathegenetic relationship is discussed.