Epstein-Barr virus (EBV) has been implicated in the development of lymphomas in immunocompromised patients. To test this hypothesis, 26 lymphomas involving the central nervous system (CNS) (11 primary, 15 systemic) were studied for the presence of EBV. In situ hybridization (ISH) was performed on formalin-fixed, paraffin-embedded tissue using a sulfur 35 (35S)-labeled EBV probe (EBV BAMH1-W). The results were interpreted without knowledge of the patients' immunologic status. The EBV sequences were detected in 11 lymphomas, nine of which were mixed or large cell subtypes. Review of the clinical information revealed that nine of the 26 lymphomas occurred in immunocompromised patients secondary to renal transplantation, human immunodeficiency virus infection, leukemia, and Wiskott-Aldrich syndrome. The EBV sequences were detected in all nine lymphomas occurring in immunocompromised patients, whereas two of the 17 lymphomas occurring in immunocompetent patients expressed EBV sequences. The authors conclude that the presence of EBV sequences in CNS lymphomas is highly correlated with a history of compromised immune status supporting a pathogenetic role of EBV in the development of CNS lymphomas in immunocompromised patients.