Epidemiologic studies have long suggested that Kaposi's sarcoma (KS) is caused by a sexually transmissible infectious agent. A new, and presumably human, herpesvirus, Kaposi's sarcoma-associated herpesvirus (KSHV/HHV-8), has been detected in KS lesions from AIDS patients by sequence-based detection techniques. KSHV is present in almost all KS lesions from all forms of KS. The virus is a Rhadinovirus or gamma-2 herpesvirus most closely related to Herpesvirus saimiri (HVS), and possesses several genes that may allow it to modify its host cell environment. KSHV has been isolated in vitro with immortalized B cell lines derived from a second malignancy associated with KSHV, body cavity-based lymphomas (BCBL). Epidemiologic studies performed to date indicate that KSHV, unlike other human herpesviruses (HHV), is not ubiquitous. The growing body of evidence indicates that KSHV is a potent oncogenic herpesvirus and the likely infectious cause of KS and BCBL.