Interferon (IFN) therapy has been proven to induce the normalization of serum alanine aminotransferase (ALT) levels and to eradicate the hepatitis C virus (HCV) in some patients with chronic hepatitis C, and these patients are usually defined as 'sustained responders'. However, there have been some reports of hepatocellular carcinoma (HCC) in these patients, and the development of HCC remains life-threatening in patients who clear HCV. We analysed the long-term prognoses of patients with chronic hepatitis C in whom HCV was eradicated with IFN. We investigated 392 sustained responders to IFN therapy, from 1,277 patients with chronic HCV infection who received IFN treatment at one of our institutions between April 1989 and March 1999. We analysed the medical records and looked for the development of HCC. About 30% of the sustained responders had been lost to follow-up 3 years after the end of IFN therapy, and the follow-up rate of sustained responders was significantly lower than that of non-sustained responders (P < 0.0001). HCC were found in eight patients: in seven patients HCC developed within 5 years after completion of IFN therapy; but in one patient, a single HCC less than 3 cm in diameter was detected between 7 and 8 years after completion of IFN. Of the five patients who had regular medical follow-up, the HCC was solitary, and the patients survived without any evidence of recurrence. Of the three patients who had not been followed-up, two died from HCC and HCC recurred in the third. These results suggest that HCC can develop in sustained responders and that sustained responders should be followed-up closely after completion of IFN so that HCC may be detected at an early stage. The optimal duration of the follow-up period of the sustained responders remains unclear. Additional prospective studies are required in order to establish an appropriate follow-up protocol for sustained responders to IFN.