The development of hepatocellular carcinoma (HCC) was significantly reduced in both sustained responders (SR) and transient biochemical responders (TR) in chronic hepatitis C (CH-C) patients who received interferon (IFN) therapy. However, the long-term clinical outcome of TR remains unclear. One thousand three hundred and seventy CH-C Japanese patients who received IFN therapy and 54 control cirrhotic patients were enrolled. TR were defined as those patients who showed a normal serum alanine aminotransferase level (<==30 IU/l) at the end of therapy and then relapsed. Mean follow-up period was 5.6 years (6.1 years in 48 cirrhotic patients) in the IFN group and 8.3 years in the 54 control cirrhotic patients. HCC was detected in 114 patients in the IFN group among whom 4 were in the 425 SR, 21 were in the 359 TR and 89 were in the 586 non-responders (NR). The cumulative incidence of HCC was significantly (P=0.0001) inhibited in both SR and TR compared with NR. Its inhibitory effect in TR was within 5 years. Platelet count did not significantly decrease for 2-4 years after IFN therapy in TR, but it significantly decreased in NR 2 years after IFN therapy. The cumulative survival in both SR and TR was significantly higher than NR (SR vs NR; P=0.0001, TR vs NR; P=0.0305). These results indicate that IFN therapy lowers the rate of the progression of HCC and improves the long-term survival even in CH-C patients who transiently respond to IFN therapy.