Kaposi sarcoma-associated herpesvirus (KSHV), or human herpervirus 8 (HHV-8), is a gamma-herpesvirus that infects human lymphocytes and is associated with primary effusion lymphoma (PEL). Currently, the role of viral infection in the transformation of PEL cells is unknown. One possibility is that KSHV, like the lymphotropic viruses Epstein-Barr virus (EBV) and human T-cell leukemia virus I (HTLV-I), activates the transcription factor NF-kappaB to promote survival and proliferation of infected lymphocytes. To examine this possibility, we assessed NF-kappaB activity in KSHV-infected PEL cell lines and primary tumor specimens by electrophoretic mobility shift assay (EMSA). We observed that NF-kappaB is constitutively activated in all KSHV-infected lymphomas, and consists of 2 predominant complexes, p65/p50 heterodimers and p50/p50 homodimers. Inhibition experiments demonstrated that Bay 11-7082, an irreversible inhibitor of IkappaBalpha phosphorylation, completely and specifically abrogated the NF-kappaB/DNA binding in PEL cells. PEL cells treated with Bay 11 demonstrated down-regulation of the NF-kappaB inducible cytokine interleukin 6 (IL-6), and apoptosis. These results suggest that NF-kappaB activity is necessary for survival of KSHV-infected lymphoma cells, and that pharmacologic inhibition of NF-kappaB may be an effective treatment for PEL.