In the last 2 years, the discovery that the suspected causative agent of Kaposi's sarcoma (KS) is a new gamma-herpesvirus, called human herpesvirus type 8 (HHV8) or Kaposi's sarcoma-associated herpesvirus (KSHV), has been followed by studies showing it to be a sine qua non of all clinical forms of KS and a specific marker for KS in the differential diagnosis of angioproliferative lesions. Reports that the virus is ubiquitous have been based on the polymerase chain reaction and appear to be contradicted by serological studies of blood donors and patients with acquired immunodeficiency syndrome (AIDS)-related and classical KS. Further serological surveys and the application of molecular probes in histological sections should resolve the issue. The recent descriptions of KSHV RNA molecules and several viral mimickers of human cytokines offer the chance to map the viral latent-lytic cycle and will change the direction of cytokine research in KS. These discoveries suggest that the increase of endemic classical KS noted in Nordic countries before the AIDS epidemic was due to spread of KSHV by unknown routes. The aggregate data should force a paradigm shift away from the notion that human immunodeficiency virus infection has a necessary role to play in AIDS-KS other than as a cause of progressive immunodeficiency.