Human immunodeficiency virus (HIV)-related body cavity-based lymphomas (BCBLs) are known to exhibit unusual clinical, immunophenotypic, and genotypic features, and have recently been found to harbor DNA sequences of a new human herpesvirus, designated Kaposi's sarcoma-associated herpesvirus (KSHV) or human herpesvirus 8 (HHV-8). The authors have encountered eight cases of HHV-8-associated BCBL in HIV-infected patients. A literature search revealed an additional 50 reported cases of HIV-related BCBL, as well as reports of several other disorders associated with HHV-8 DNA. Comprehensive analysis of the clinical and pathobiological features of all 58 known cases of HIV-related BCBL shows it to be a unique B-cell neoplasm with a strong propensity for body-cavity involvement without mass lesions and with little or no dissemination, poor prognosis, high grade usually immunoblastic morphology, late B-cell phenotype and genotype, no associated c-myc gene rearrangement, frequent presence of Epstein-Barr virus (EBV) genome, and uniform association with HHV-8 DNA. Considering these features in the context of other disorders associated with HHV-8 DNA, HHV-8 appears to play a causal role in BCBL, possibly in concert with EBV, and may induce this lymphoma through dysregulation of cytokines, particularly interleukin-6, or infection of an unusual B-cell subset. The characteristics of HHV-8-associated BCBL suggest a possible role for antiherpes or anticytokine agents in the treatment of this lymphoma.