We examined the effect of the hepatitis C virus (HCV) peptide vaccine IC41 on HCV-specific T-cell responses and virological relapse rates in patients with chronic HCV genotype 1 infection when added to pegylated interferon plus ribavirin standard therapy. 35 patients received 6 vaccinations with IC41 from weeks 28 to 48 of standard antiviral treatment and were followed-up for another 6 months. IC41 vaccination did not prevent HCV-RNA relapse in patients with ongoing interferon standard treatment but HCV-specific T-cell responses were inducible and were associated with lower relapse rates. An increase of HCV-specific T-cell responses occurred in 73% of patients, responses were more frequent and stronger in patients with sustained virologic response than in patients who relapsed. Optimized vaccine responses may enhance sustained virologic response rates obtained with standard treatment of chronic hepatitis C.