Hepatitis C virus (HCV)-specific cytotoxic T lymphocytes (CTL) have been shown to play a role in host defense and pathogenesis of chronic HCV infection. Our aim was to test the hypothesis that intrahepatic HCV-specific CTL activity may impact subsequent response to interferon alfa (IFN-␣) therapy. Of the 37 patients that we have prospectively evaluated for HCV-specific CTL activity in liver, 21 received IFN therapy, and 19 completed a 6-month course and attended 6 to 18 months of follow-up. Intrahepatic CD8 ؉ cells were isolated from liver biopsy tissue and tested against target cells expressing HCV antigens to determine intrahepatic CTL activity. The relationship between treatment response and HCV-specific CTL activity and other factors known to associate with response (genotype, viremia, histology) was analyzed. HCV-specific CTL activity was detected in 9 of 21 patients (and 9 of 19 who completed therapy). After 6 months of IFN therapy, 8 of 19 (42%) patients had normal serum alanine transaminase (ALT) (complete responders). After 18 months of follow-up, only 3 patients (16%) had a sustained biochemical response. Of the 9 patients with detectable HCV-specific CTL activity in their liver before treatment, 7 (78%) developed a complete response. In contrast, only 1 of the 10 patients with no detectable HCV-specific CTL activity developed a complete response to IFN (P F .01). In 6 of 8 patients with a complete response, including the 3 sustained responders, the CTL response appeared to be directed predominately to the HCV core region. These data suggest that the host immune response, particularly that mediated by CD8 ؉ CTL, may be important in determining the outcome of IFN therapy for chronic HCV infection. Further understanding of the mechanism of action of IFN should impact the design of better therapeutic strategies against chronic HCV infection. (HEPATOLOGY 1998;28:225-230.)The host cellular immune defense, including the CD4 ϩ and CD8 ϩ T-cell response, is activated in patients with hepatitis C virus (HCV) infection. The intensity of the T-cell response during the early stages of infection may be a critical determinant of disease resolution and control of infection. 1,2 In chronic viral infection, viral-specific CD8 ϩ cytotoxic T lymphocytes (CTL) have been shown to be important in host defense and in the control of the viral infection. [3][4][5][6][7] In lymphocytic choriomeningitis virus infection, virus-specific CTL activity reduces viral titers by 4 to 5 logs. 8 It is felt that CTL might lead to viral clearance through either direct lysis of infected cells or cytokine-mediated mechanisms. We and others have shown that HCV-specific CTL activity is detectable in the liver in a proportion of patients with chronic HCV infection. The presence of HCV-specific CTL activity in bulk-expanded CTL was found to be associated with a lower level of viremia and a greater inflammatory activity in the liver, suggesting that HCV-specific CTLs may assist in the control HCV infection, but, in so doing, contribute to hepatoce...