The activity of interferon (IFN) is not elucidated from the viewpoint of cancer prevention in chronic hepatitis C patients en masse. The hepatocellular carcinogenesis rate was analyzed statistically in 1,643 patients with chronic hepatitis C: 1,191 patients with IFN therapy and 452 without IFN therapy. Hepatocellular carcinogenesis rates in the treated and untreated groups were 2.1% and 4.8% at the end of the 5th year, and 7.6% and 12.4% at the 10th year, respectively (P =.0036). Multivariate analysis showed that IFN slightly decreased the risk of carcinogenesis by 33%, compared with that of untreated patients (P =. 14), adjusting for the confounding effects of age, fibrotic stage, gender, and gamma-glutamyl transpeptidase (GGTP) value. Among 1,191 patients with IFN, 461 patients attained persistent loss of hepatitis C virus (HCV) RNA, and the other 145 patients retained normal alanine transaminase (ALT) values without loss of HCV RNA. The hazard of carcinogenesis in these 606 patients with persistent normal ALT with or without HCV-RNA clearance was significantly lower than that of untreated patients (hazard ratio: 0.32; P =.012) and that of the abnormal aminotransferase group. Among patients with chronic hepatitis C, IFN significantly decreased the hepatocellular carcinogenesis rate in those patients with normal or persistent low ALT values.