We searched for Epstein-Barr virus (EBV) sequences by enzymatic DNA amplification in nine primary brain lymphomas from patients without immunodeficiency. We used seven nonlymphoma brain tumors as negative controls, and the Raji cell line as a positive control. We detected EBV DNA, using ethidium bromide-stained-agarose minigel electrophoresis and dot blot hybridization, in the positive control and in only one brain lymphoma tumor; we did not detect EBV DNA in the other tumors. The EBV-positive patient had a second B-cell monoclonal population in the peripheral blood without detectable EBV DNA, suggesting a direct role for EBV in the development of the brain lymphoma.