Epidemiological studies to determine the prevalence of rheumatoid arthritis (RA) have been carried out in two South African Negro populations: one rural people in the north-western Transvaal, and the other an urban community in Johannesburg. Altogether 1352 subjects over the age of 15 years were examined clinically and radiologically; serological tests for rheumatoid factor (RF) were carried out in 920. Rheumatoid arthritis was graded 'definite' or 'probable' on the basis of a modification of the 'Rome criteria' of 1963. A marked difference in the prevalence of RA was encountered in the two populations. Among the rural Blacks only 1 respondent had definite RA and 6 had probable RA, giving a prevalence of 0,87% for combined definite and probable RA. Among the urban group 5 respondents had definite RA and 13 had probable RA, a combined prevalence of 3,3%. This difference is statistically highly significant (P less than 0,01). Moreover, the form and severity of the disorder differed markedly in the two populations. In the rural community such changes as were encountered were invariable mild and no-one had clinical features resembling classical rheumatoid arthritis; by contrast, among the urban Blacks the changes resembled those of rheumatoid disease in White populations. Such marked differences in genetically closely related communities point to the importance of sociological and environmental factors in the pathogenesis of rheumatoid arthritis.