Kaposi's sarcoma (KS) was recognized in 1948 in Zaire, where it has probably always been endemic. In 1957 a relative frequency of KS of 9% of all biopsied cancers was found. There are fluctuations in incidence within the country, with a higher incidence in the east, where it was estimated in 1960 at about 5-10 cases per 100 000 per year in males, with a relative frequency of 14% of all malignant male tumours and a M/F ratio higher than 10/1. More recently KS accounted for roughly 17% in males and 2% in females of all malignant biopsied tumours in north-east Zaire (1969-1983). In 2 years (1982-1983), 26 male and 5 female KS cases were histologically confirmed in an area of eastern Zaire with a population of roughly 300 000 people. It has been suggested, on the basis of this high incidence of KS and of the recent identification of African AIDS cases, that the hypothetical transmissible agent in AIDS might originate from Central Africa. The frequency of KS in African AIDS cases was low (16%) as compared to that in the USA. The M/F ratio of AIDS was 6/4 and that of AIDS-associated KS 1/1. AIDS-associated cases occurred in young adults and were generalized and fulminating. African KS occurs at a younger modal age than in Caucasians, which is the combined result of increasing incidence with age and of the high proportion of young people in the population in Africa but not of a higher risk for younger male adults, as in AIDS.(ABSTRACT TRUNCATED AT 250 WORDS)