HHV8 (KSHV) is a new member of the herpesvirus family, closely related to EBV and HVS saimiri, recently isolated from Kaposi sarcoma (KS) tissue. It is a lymphotropic virus that infects B cells in humans. It is transmitted mainly by sexual intercourse and not through blood or from mother to child. HHV8 infects more frequently homosexuals and individuals with multiple partners. The rate of seroprevalence rises with homosexuality and HIV infection, mainly in individuals infected by homosexual contact and in Africa. Haemophiliacs, intravenous drug users and women infected by HIV have a seroprevalence rate identical to that of healthy blood donors. HHV8 is more frequent in certain regions, mainly southern Europe (southern Italy, Greece) and in East and Central Africa. It is found in all types of KS, including those encountered in AIDS. It is associated with two lymphoproliferative disorders, primary effusion lymphoma (PEL) and certain cases of multicentric Castleman's disease (MCD). PEL are rare and aggressive non-hodgkin's lymphomas (NHL) of B cell lineage that have a propensity for body cavities, with malignant effusions. They do not express normal B cell phenotype, but often express CD45 and CD30. The immunoglobulin genes show a monoclonal rearranged pattern. Lymphoma cells have a morphology that bridge that of immunoblastic and anaplastic cells. c-myc oncogene always displays a germline configuration, a feature that differs from other NHL with serous involvement. EBV is often associated to HHV8 in these cells. They occur more frequently during AIDS but some cases affecting non HIV infected individuals are reported. MCD is an atypical lymphoproliferative disease with marked vascular hyperplasia, with constitutional symptoms. It can occur in the course of HIV infection. In these cases, it is almost always associated with HHV8 infection, and affected patients can develop KS. HHV8 is also found, although less frequently, in MCD affecting non HIV infected individuals. The role of HHV8 in other lymphoproliferative diseases is under investigation.