Several studies have shown that the frequency of hepatitis C virus (HCV) infection is high in patients with B-cell non-Hodgkin's lymphoma (NHL). In these studies, liver dysfunction during chemotherapy has been demonstrated, but changes in HCV ribonucleic acid (RNA) levels during chemotherapy have not been well documented. In this study, we monitored serum HCV RNA levels and liver function in five HCV-infected patients with B-cell NHL undergoing treatment with rituximab-combination chemotherapy. Increased HCV RNA levels during or after the chemotherapy were observed in all five patients, and a significant increase in transaminases was seen in one case. In this case, serum HCV RNA level dramatically decreased at the time of the increase of transaminases, and this suggested that the cause of liver damage was an immune reaction against hepatocytes with HCV and not any anticancer drug induced liver toxicity. Monitoring of serum HCV RNA levels and transaminases may be helpful to understand the cause of liver dysfunction in patients receiving chemotherapy. However, increases of HCV viral load were not associated with the occurrence of liver dysfunction in this study. Further studies will be necessary to investigate more fully the relationship between changes in HCV viral load and liver function during chemotherapy for HCV-infected patients. Am. J. Hematol., 2008. © 2007 Wiley-Liss, Inc.