Hepatitis C virus (HCV) is both hepatotropic and lymphotropic and has been shown to be associated with the benign lymphoproliferative disorder, mixed cryoglobulinemia (MC). Preliminary studies suggest that there may be an association between chronic hepatitis C, MC, and non-Hodgkin's lymphoma (NHL). The aim of this study was to determine whether patients with chronic HCV and MC have occult bone marrow NHL. Sixteen patients with chronic HCV and clinically active MC underwent bone marrow biopsy and aspiration. Flow cytometry was performed looking for abnormal B-cell lineage. Molecular genetic studies were performed to identify B-cell monoclonality. Nine of 16 patients (56%) had abnormal marrow morphology, 7 (44%) were interpreted as suspicious for lymphoma, and 2 (13%) as consistent with lymphoma. Flow cytometry on 13 patients identified5 (39%) with increased B-cell populations. Molecular analysis on 13 patients identified 3 (23%) with monoclonal proliferation ofthe B-cell lineage. All 13 patients tested for Epstein-Barr virus were negative by polymerase chain reaction (PCR). Four of 16 patients(25%) had marrow morphology and evidence of a monoclonal B-cell population by flow cytometry and/or molecular studies, consistent with B-cell NHL. These findings confirm the presence of lymphoproliferative disorders in patients with chronic HCV and MC. Some of these disorders meet the morphological and molecular criteria for the diagnosis of malignantlymphoma