A population-based epidemiologic study of 2,184 residents of Charleston County, South Carolina, with a high morbidity and death rate from arteriosclerotic and hypertensive cardiovascular disease showed high prevalence of hypertension among Negroes compared with the prevalence in Charleston whites or American Negroes. The relationships of sex and age with hypertension follow different patterns in Negroes and whites. In contrast to the white population, no correlation between obesity and hypertension was found in Negroes. A positive correlation was found between skin pigmentation and hypertension in Negroes. There was also a significant difference in prevalence of group B blood type between lighter- and darker-pigmented Negroes, the darker half having the same "B" prevalence as West African Negroes, and 50% higher prevalence than American Negroes.