The epidemiologic findings of Kaposi's sarcoma (KS) among patients with the acquired immunodeficiency syndrome (AIDS) suggest that human immunodeficiency virus (HIV) infection is insufficient for the development of KS. It was speculated that another sexually transmitted infection is responsible for the markedly increased incidence of KS among patients who acquired HIV infection through sexual intercourse. However, no such contributing infectious agent was consistently identified. The canine transmissible venereal tumour (TVT) is a malignant tumour that can be transplanted by viable cells across major histocompatibility complex (MHC) barriers. Recent findings suggest that all canine TVTs originated from the same tumour and were transferred from one animal to the other during sexual intercourse. It is suggested that, in analogy with the canine TVT model, the characteristics of KS epidemic among AIDS patients may be explained by transmission and engraftment of viable malignant cells during intercourse.