Although epidemiologic and experimental data suggest an etiopathogenetic role for both hepatitis C virus (HCV) and Epstein-Barr virus (EBV) infection in development of B-cell non-Hodgkin's lymphoma (NHL), potential interactions between EBV and HCV during progression of B-cell NHL have not yet been fully investigated. In the present study, tumor biopsy specimens from patients with both B-cell NHL and chronic HCV infection (HCV(+)) were analyzed for the presence of EBV-encoded RNA (EBER) by in situ hybridization (ISH). VH and VL gene segments were amplified from tumor biopsy specimen DNA by PCR. EBV infection (EBV(+)) was detected in tumors from 2 of 31 (6%) HCV(+) B-cell NHL patients. Clinical histories of these two EBV(+)/HCV(+) B-cell NHL patients indicated a particularly aggressive course of disease. Chemotherapy failed to induce long lasting remission for either of these EBV(+)/HCV(+) B-cell NHL patients. Amplification of CDR3 of the Ig heavy chain gene from DNA isolated from each EBV(+)/HCV(+) B-cell NHL indicated the presence of monoclonal B-cell expansion. Rearrangement of Ig genes in neoplastic B-cell clones from both EBV(+)/HCV(+) patients was similar to that previously reported for EBV(-)/HCV(+) B-cell NHL patients. Additionally, neoplastic B-cell clones from these two EBV(+)/HCV(+) B-cell NHL patients did not exhibit intraclonal variation. Previous studies have demonstrated that intraclonal variation is common among neoplastic B-cell clones from EBV(-)/HCV(+) patients. EBV infection may have prevented evolution of variant neoplastic B-cell clones by suppressing antibody affinity maturation. Together, these data suggest that EBV infection may cooperate with HCV infection during progression of B-cell NHL in immunocompetent individuals. Such an interaction may accelerate the course of disease in B-cell NHL patients.