Tissue specimens from 21 cases of Hodgkin's disease were examined for the presence of Epstein-Barr virus (EBV) and cytomegalovirus DNA by molecular hybridization techniques. EBV DNA was detected in 4 cases, including 2 of 8 cases which had been previously shown to contain clonal immunoglobulin gene rearrangements. Two of the cases containing EBV DNA were of the nodular sclerosing type and had received prior therapy; the other 2 were classified as mixed cellularity Hodgkin's disease and had not received therapy before the biopsy tissue was obtained. Analyses of the terminal portions of EBV genomes indicated a monoclonal or oligoclonal proliferation of EBV-infected cells in the tissues studied. In contrast, none of the 21 cases had detectable cytomegalovirus DNA sequences. The identification of EBV DNA may reflect the proliferation of lymphoblastoid cells due to the reduced immune competence frequently noted in Hodgkin's disease or may indicate the presence of EBV genomes within Reed-Sternberg cells.