Human herpesvirus 8 (HHV-8, KSHV) is a novel virus for which fragments of genomic sequence were identified in 1994. This was done by means of a differential amplification technique applied to the DNA of Kaposi's sarcoma lesions and normal tissues obtained from the same individual. The analysis of nucleotide sequence has shown that HHV-8 is closely related to herpesvirus saimiri and Epstein-Barr virus, two members of Gammaherpesvirinae sub-family. The virus has been observed by means of electron microscopy in chronically infected cell lines. Serological studies are still preliminary but seem to demonstrate a low prevalence of HHV-8 infection among the general population at least in Western countries, HHV-8 is mainly detected by means of polymerase chain reaction (PCR) and molecular hybridization. HHV-8 detection in human tissues is strongly associated with three diseases: Kaposi's sarcoma, body-cavity-based lymphomas and Castleman's disease. The demonstration of the causative role of HHV-8 in the occurrence of these three diseases, particularly Kaposi's sarcoma, would have major consequences for their diagnosis and treatment.