Human herpesvirus type 8, or Kaposi's sarcoma-associated herpesvirus (KSHV), is the only known human g(2) herpesvirus (rhadinovirus) and the most recently discovered tumor virus. KSHV is associated with Kaposi's sarcoma and two other lymphoproliferative disorders in the AIDS setting: primary effusion lymphoma and the plasma cell variant of multicentric Castleman's disease. This review offers an update on the epidemiology and the role of KSHV genes in the development of disease.