The human herpes virus 8 (HHV8) or Kaposi's sarcoma-associated herpes virus (KSHV) is present in all Kaposi's sarcoma, and the detection of the virus using polymerase chain reaction or in situ hybridization is a highly sensitive and specific diagnostic test for the diagnosis of this neoplasm. HHV8 is furthermore invariably present in primary effusion lymphoma (PEL) and has also been detected in patients with acquired immunodeficiency syndrome (AIDS)-associated multicentric Castleman's disease (MCD) as well as, to a lesser extent, in non-AIDS MCD. In contrast to Kaposi's sarcoma, in which the tumor cells show primarily latent HHV8 infection, a higher rate of lytically infected cells can be observed in MCD. Epidemiological surveys indicate that the seroprevalence for HHV8 parallels the risk of developing Kaposi's sarcoma--5-10% in the general population of the Western world but ranging up to 20-70% in homosexual human immunodeficiency virus (HIV)-infected patients, and the infection precedes the development of Kaposis's sarcoma. Finally, HHV8 has been reported in a number of other diseases, especially in multiple myeloma. However, the highly controversial role of HHV8 in these lesions has to be clarified. Based on the data available today, HHV8 can be assigned as a new human virus, associated with tumors.