A study of arterial circles of Willis from a series of 715 consecutive autopsies in Ugandan Africans shows that severity of cerebral atherosclerosis increases with age. No significant difference exists between men and women nor among main tribal groups. The severity is less in Ugandan Africans than in United States blacks but is approximately the same in United States Caucasians. However, the Ugandan Africans show a more severe cerebral atherosclerotic involvement than do the Nigerians.