A review of the cases of Moyamoya disease at two large Mid-Western United States Universities was undertaken for the purpose of assessing the epidemiology of Moyamoya disease. A total of 51 cases of Moyamoya disease were identified, with 12 cases classified as akin Moyamoya disease, nine cases of probable Moyamoya disease, and 30 cases of classic or definite Moyamoya disease. The conditions associated with akin Moyamoya were sickle cell disease, Down's syndrome, trauma, radiation, and neurofibromatosis. The mean age of presentation for probable and classic Moyamoya disease was 22 years. The sex predilection was approximately equal, with a slight female predominance. The racial background was identified in 22 of the definite cases, and included six patients with oriental inheritance, three with American Indian inheritance, one black, and the remainder Caucasian. Of some interest, there were five Caucasian patients with names identifiable as Eastern European in origin. The mean age of presentation of the definite Moyamoya disease was 14 years, the probable Moyamoya disease was 4 years, and the akin Moyamoya disease was 5 years.