Diseases of urban and rural Blacks in South Africa are reviewed. In rural Blacks the major problems are infection and malnutrition. Other important disorders include cancer of the oesophagus, liver and cervix, and rheumatic heart disease and cardiomyopathy. The diseases in urban Blacks are those of a population in transition. Characterised by all gradations of socioeconomic development, from the relatively primitive to the completely westernised, these people exhibit a correspondingly wide and varied range of disease embracing the afflictions of rural dwellers and the new diseases of the city. Whereas the prevalence of some of the former, such as infection and malnutrition, is declining, they still constitute a considerable problem in urban Blacks. More important is the increasingly serious impact of the new disorders, which may be divided into two groups: (a) a large range and variety of alcohol-related disorders with serious effects at the social, economic, psychological and physical levels; and (b) most, if not all, of the diseases encountered in western populations. Some of these, such as obesity and hypertension, have not only attained epidemic proportions among urban Blacks, but their prevalence may actually have exceeded that among Whites. Other conditions, such as coronary heart disease, gout, gallstones and colonic cancer, which emerged later, are relatively uncommon or rare. A plea is made for much greater epidemiological research. This is necessary in order to obtain reliable knowledge of the prevalence of disease, to determine the best ways of applying present knowledge with existing and future resources, and to obtain knowledge regarding both old and new diseases of which the pathogenesis is still obscure.