Kaposi's sarcoma-associated herpes virus (KSHV), also known as human herpes virus-8 (HHV-8), is a recently described gamma-herpes virus that has been etiologically linked to two different acquired immunodeficiency syndrome (AIDS)-related malignancies by strong epidemiologic and pathologic evidence. Infection been shown to precede and predict the development of Kapasi's sarcoma (KS) in human immunodeficiency virus (HIV)-infected patients, and viral DNA has been found in KS lesions of all types and stages. Furthermore, KSHV is a lymphotropic virus and is present in nearly all cases of primary effusion lymphoma, a rare malignancy disproportionately affecting HIV-infected individuals. KSHV is also thought to dramatically affect the incidence, type, and course of multicentric Castleman's disease, another lymphoproliferative disorder over-represented in people with AIDS. KSHV encodes many potentially oncogenic products, including several apparently pirated from the human genome. These include various chemokines, cell cycle regulatory proteins, and survival and proliferation factors. Knowledge is rapidly accumulating concerning the viral pathogenic mechanisms and host cofactors necessary for KSHV-mediated disease.