The authors report a rare case of hepatocellular carcinoma (HCC) that developed 6 years after a sustained and complete response to interferon (IFN) therapy for chronic hepatitis C. A 61-year-old Japanese man presented with a mass in the liver that was diagnosed as HCC. Six years earlier he was treated with IFN-alpha and responded successfully to therapy, with sustained normalization of serum aminotransferases and eradication of serum hepatitis C virus (HCV)-ribonucleic acid (RNA). HCV-RNA was also not detected in the resected tumorous and nontumorous liver tissues. The findings suggest that all patients with chronic HCV infection should be followed closely for as long as possible for the potential development of HCC even after a complete and sustained response to IFN treatment.