Kaposi's sarcoma-associated herpesvirus (KSHV) or human herpesvirus 8 (HHV-8) is present in all clinical variants of Kaposi's sarcoma and two lymphoproliferative diseases: primary effusion lymphoma and multicentric Castleman's disease (MCD). Seroepidemiological studies have proved the link between KSHV/HHV-8 infection and development of Kaposi's sarcoma. The virus encodes a number of genes homologous to human genes involved in cell proliferation, anti-apoptosis, angiogenesis and cytokine action. In Kaposi's sarcoma and primary effusion lymphoma, however, most tumour cells only show evidence of latent infection, with a restricted gene expression pattern including latency associated nuclear antigen 1, able to interact with p53 and the retinoblastoma gene. The small number of tumour cells expressing lytic genes may, however, be important for tumour growth by paracrine mechanisms as has been shown for the viral G-protein-coupled receptor. Understanding these molecular mechanisms would provide the rationale for new treatments and prevention of KSHV/HHV-8-associated diseases.