The occurrence of Kaposi's sarcoma (KS) in patients with HIV infection is more than 7,000 times higher than in the non-HIV infected population. The reason for this association is unclear but may involve decreased immune surveillance as a result of the profound cellular immune deficiency caused by HIV, a sexually transmitted KS-inducing virus, whose KS-transforming capabilities may be enhanced by HIV, or a direct or indirect effect of HIV itself in susceptible individuals. Over the last few years, advances have been made in our understanding of the pathogenesis of this tumor, and several models have been proposed for its development in the setting of AIDS. Better characterization of the processes involved in the development of KS will ultimately lead to more effective methods of treating and preventing this unusual tumor.