Kaposi's sarcoma-associated herpesvirus (KSHV, also known as human herpesvirus 8, or HHV 8) is a virus that is consistently present in Kaposi's sarcoma^1,2 and in primary-effusion (body-cavity-based) lymphomas³, malignancies that occur frequently, but not exclusively, in AIDS patients. KSHV is a gamma herpes-virus with homology to herpesvirus Saimiri and Epstein-Barr virus^1,4, both of which can transform lymphocytes⁵. Cloning of a KSHV genome fragment revealed the presence of an open reading frame encoding a putative G-protein-coupled receptor⁶ that is homologous to a G-protein-coupled receptor encoded by herpes-virus Saimiri^7,8 and to human interleukin-8 receptors^9,10. Here we show that the KSHV G-protein-coupled receptor is a bona fide signalling receptor which has constitutive (agonist-independent) activity in the phosphoinositide–inositoltrisphosphate–protein kinase C pathway. Furthermore, the KSHV G-protein-coupled receptor stimulates cellular proliferation, making it a candidate viral oncogene.