Hepatitis C virus (HCV) infection is associated with chronic hepatitis, cirrhosis, and hepatocellular carcinoma. Recent evidences have also suggested that HCV infection contributes to development of autoimmune disorders and B-cell nonHodgkin's lymphoma (NHL). Mechanisms by which HCV infection promotes B-cell NHL development remain unclear. Increased serum osteopontin (OPN) levels have been associated with several autoimmune diseases as well as a variety of cancers. However, the association between OPN and B-cell NHL or HCV-associated B-cell proliferation has not previously been reported. In the present study, we determined whether serum OPN differences were associated with HCV infection, type II mixed cryglobulinemia (MC) syndrome and B-cell NHL. Serum OPN levels were measured by capture enzyme-linked immunosorbent assay. Our results show that high serum OPN levels are associated with B-cell NHL and HCV infection. Interestingly, highest serum OPN concentrations were found among HCV-infected patients with concomitant type II MC syndrome with and without B-cell NHL. These data indicate that OPN is involved in the lymphomagenesis, especially, in the context of HCV infection and autoimmune diseases.