Because of the various neoplastic manifestations of human immunodeficiency virus (HIV) and the variable period between HIV infection and the development of tumors related to acquired immunodeficiency syndrome (AIDS), it is possible that certain behaviors, toxins, genes, or infectious agents--particularly viruses--may act as cofactors in the pathogenesis of AIDS-related neoplasms. Most epidemiologic and laboratory investigations of possible cofactors have been directed toward Kaposi's sarcoma (KS), by far the most common AIDS-related tumor and one closely associated with male homosexual lifestyle in the U.S. Nonetheless, epidemiologic investigations of putative associations have not demonstrated any clear association between KS and particular viruses. Furthermore, laboratory investigations, both serologic and molecular/genetic, have failed to definitively implicate as cofactors for KS these viruses: cytomegalovirus, Epstein-Barr virus (EBV), herpes simplex viruses, pathogenic human papillomaviruses, or human herpes virus type 6. Investigations of a suggested association between EBV and AIDS-associated non-Hodgkin's (B cell) lymphomas (NHLs) have also been inconclusive. However, HIV may act as a cofactor in accelerating the development of hepatitis B-associated hepatocellular carcinoma. In summary, viral or other cofactors have not been definitely identified as cofactors in AIDS-related tumors.