In a house-to-house study of 994 urban Zulus the prevalence of hypertension according to WHO criteria was 25% (23% men, 27% women). In a rural Zulu study of 987 the prevalence age corrected to the urban distribution was 9.4% (8.7% men, 10% women). Thus there was a pronounced difference between the prevalence of hypertension in the urban and rural Zulu (p less than 0.0005). There was an earlier onset of hypertension in the urban compared with the rural Zulu. Contrasting biosocial factors in the urban and rural Zulu could explain the differences in prevalence. Our study suggests that hypertension is not a major health problem in rural Zulus. Large-scale case finding and intervention programmes should be confined to the urban black population of South Africa.