During the course of simian immunodeficiency virus (SIV) infection, nearly 15% of rhesus macaques (Macaca mulatta) and up to 40% of cynomolgus macaques (Macaca fascicularis) developed SIV-associated non-Hodgkin's lymphomas. Most of these malignant lymphomas harbored lymphocryptoviruses, which are closely related to the human Epstein-Barr virus (EBV; Herpesvirus M. mulatta and Herpesvirus M. fascicularis). To characterize the oncogenic role of simian EBV infection for lymphomagenesis during SIV infection, expression of the EBV-encoded latent membrane protein-1 (LMP-1) was analyzed in malignant lymphomas of SIV-infected rhesus macaques. Nine seropositive rhesus macaques suffering from B-cell lymphomas during the late phase of SIV infection were euthanized. Latency stages of EBV infection within malignant lymphomas and simian EBV-infected lymphoblastoid cell lines (LCL8664, H50) were characterized by analyzing expression of the EBV-encoded nuclear antigens EBNA-1, EBNA-2, and small RNAs EBER1/2. In parallel, the presence of viral LMP-1 transcripts was assessed by reverse transcription-polymerase chain reaction (RT-PCR) and in situ hybridization. Results were compared with findings in AIDS-associated malignant lymphomas in two patients infected with human immunodeficiency virus (HIV)-1. Rhesus macaques developed high-grade B-cell lymphomas of the centroblastic (five of nine), immunoblastic (two of nine), centroblastic-centrocytic (one of nine), and Burkitt-like (one of nine) subtypes within 18-29 months postinfection with SIV(mac)251/32H. The presence of Herpesvirus M. mulatta was detected in eight of nine cases. Transcription of the viral oncogene LMP-1 could be demonstrated within the simian EBV-infected cell lines as well as in four of nine SIV-associated malignant lymphomas. These four cases and both of the HIV-1-related non-Hodgkin's lymphomas expressed the full spectrum of latent EBV gene products (LMP-1, EBER1/2, EBNA-1, EBNA-2) and were thus classified as latency type III stages of EBV infection. Simian EBV infection was demonstrated in 90% of lymphomas in SIV-infected rhesus macaques. Analysis of LMP-1 expression suggests an important role for this viral oncogene in the pathogenesis of both SIV and HIV-1-associated malignant lymphomas.