The study of a patient suggested a relationship between Epstein-Barr virus infection and primary lymphoma of the central nervous system in the acquired immune deficiency syndrome. Deoxyribonucleic acid preparations from tumor tissue contained 30 to 100 copies of Epstein-Barr virus genome per cell when hybridized with a probe consisting of the Bam-HI K fragment of Epstein-Barr virus strain FF41. This hybridization study suggests that induction of this patient's central nervous system lymphoma was related to Epstein-Barr virus infection.