To determine the virological factors associated with a favorable long-term response to interferon-alpha (IFN) therapy in chronic hepatitis C virus (HCV) infection, 61 Japanese patients with chronic HCV infection were treated with IFN for 24 weeks (780 million units in total) and followed for 8 to 16 months after cessation of therapy. Ten patients dropped out because of severe side effects. Of the 51 patients who completed IFN therapy, 23 showed complete and sustained response (CR --> SR), 13 complete response with early relapse (CR --> Rel), and 15 no response to IFN (NR). For the pretreatment serum HCV RNA level, 20/23 who had CR --> SR had < 10(6) eq/ml compared to 3/13 CR --> Rel and 1/15 NR (P < 0.01). Serologically defined HCV type 2 infection was also associated with a better opportunity to develop CR --> SR compared to CR --> Rel of NR (P < 0.01). Loss of serum HCV RNA at week 4 of IFN therapy was also associated with a more favorable long-term response [17/19 CR --> SR were HCV RNA negative compared to 3/11 CR --> Rel (P < 0.01) and 2/13 NR (P < 0.01)]. In contrast, normalization of serum alanine aminotransferase (ALT) levels at week 4 was found in 9/19 CR --> SR compared to 8/11 CR --> Rel (P = NS), and 0/13 in NR (P < 0.01). Six months after cessation of IFN therapy, 3/25 CR --> SR patients were HCV RNA positive despite normalization of serum ALT levels. These data indicated that in addition to pretreatment serum HCV RNA levels and HCV type, the kinetics of response to IFN (at week 4) were also predictive of subsequent long-term response to IFN in patients with chronic HCV infection.