Hepatitis C virus (HCV) is both hepatotropic and lymphotropic and a clear-cut association has been proposed between HCV infection and mixed cryoglobulinemia (MC), a benign lymphoproliferative disorder, which sometimes evolves into a frank malignant B cell non-Hodgkin's lymphoma (B-NHL). Moreover, in the presence of antibodies to HCV, as well as of HCV-specific genomes has been reported in the sera of over 37% patients with B-NHL, not associated with MC. Thus, we decided to perform both a serologic and a molecular study to give insights into a possible relationship between HCV infection and neoplastic lymphoproliferations. We used ELISA and RIBA tests to show that anti-HCV antibodies were present in the serum of 29 out of 69 unselected B-NHL patients (42%), while seropositivity in a healthy population was about 1%. The prevalence of anti-HCV antibodies was low in definite subsets of B lymphoid disorders, including multiple myeloma, Waldenström's macroglobulinemia and monoclonal gammopathies of undetermined significance. Then, using reverse transcriptase polymerase chain reaction, we detected HCV sequences directly in the pathologic lymph node biopsies in 13 out of 34 B-NHL cases, and in particular in six out of eight low-grade lymphomas of MALT type and in five out of eight centroblastic-centrocytic follicular lymphomas. In contrast, the peripheral blood samples from 10 B cell chronic lymphocytic leukemia patients resulted negative for the presence of HCV genomes. Similarly, viral sequences were absent in 10 T cell NHL, while only one out of the 14 Hodgkin's disease cases tested resulted positive. Finally, we used a PCR-based assay to characterize the genotypes (I-IV) present in the positive lymphomatous tissues. The presence of both serologic and molecular markers of HCV infection in a high percentage of certain types of B-NHL, not associated with cryoglobulinemia, and its absence from other lymphoproliferative diseases extends the spectrum of HCV-associated lymphoproliferations arguing in favor of some role of this viral infection in the pathogenesis of the malignant proliferation of definite B lymphoid populations.