Studies of migrants and comparisons of rural versus urban communities are potentially informative study designs because they allow examination of genetically similar population subgroups exposed to diverse environmental conditions. These designs have been underused in Africa, where recent urbanization has created many situations in which nearby communities of common ethnicity and culture live under different social and economic circumstances. The International Study of Hypertension in Blacks (ICSHIB) conducted several overlapping surveys in Nigeria starting in 1993. These surveys were based primarily in the rural village of Idere and the urban community of Idikan, both inhabited by people defined ethnically as Oyo Yoruba and sharing a common language and culture. Survey teams collected standardized blood pressure and anthropometric measurements, and some study participants provided 24-hr urine samples and questionnaire data on psychosocial stress and social integration. Rural and urban groups differed substantially in blood pressure and related characteristics. Age-adjusted prevalence of hypertension (blood pressure > or = 140/90 mm Hg) for participants aged 25 years and older was 7-8% in Idere and 24-27% in Idikan. The distributions of overweight, sodium/potassium ratio, perceived stress, and social integration scores all contributed to lower hypertension risk in Idere. The effects and interactions of these identified risk factors remain poorly understood, even among people who share a common genetic background, similar diet, and many other lifestyle features. Nonetheless, the rural-urban distinction is sufficiently salient to engender a nearly threefold difference in hypertension prevalence. This disparity in disease prevalence demonstrates the sensitivity of human beings to the environmental determinants of disease and provides a sobering example of the difficulty in identifying subtle genetic effects, which can be easily overwhelmed by small differences in environmental exposures.