It has been recently hypothesized that the hepatitis C virus (HCV) might be involved in the pathogenesis of malignant B-cell non-Hodgkin's lymphomas (NHL). On the basis of this observation we sought to determine the prevalence of HCV infection in the patients affected by B-cell NHL and extended our analysis to all the patients affected by lymphoproliferation disorders seen at our institution in the last 30 months. Five hundred and thirty-seven unselected, consecutive patients were studied. HCV infection was investigated through detection of anti-HCV antibodies and HCV-RNA. HCV genotyping was performed on HCV-RNA positive specimens. The risk of being infected by HCV was compared with that of the general population of our area. Among all lymphoproliferative disorders, the prevalence and the relative risk (RR) of being infected by HCV were increased only among B-cell NHL (9%; RR 3.24; p &lt; .0001). Among these, a strong prevalence of HCV was found only in the subgroup of immunocytomas (30%; RR 10.27; P &lt; .0001), while other histotypes were associated with it only occasionally. Because HCV-positive lymphomas clinically behave as essential mixed cryoglobulinemia (EMC), the close association between HCV infection and EMC is confirmed, and evidence is provided that the pathological substrate of EMC corresponds to the immunocytoma. HCV genomic sequences were found in 84% of patients analyzed. Viral genotypes were those more frequent in our area.