Acute infection with hepatitis C virus (HCV) develops into a chronic hepatitis in about 50-70% of patients. Treatment of these patients with interferon-α (IFN-α) results in a sustained long-term response in only 15-20% but causes numerous unwanted side-effects in a higher percentage of patients. The aim of our study was to define host or viral parameters that would allow identification of responders and non-responders to IFN-α prior to the onset of treatment. We studied a group of 87 patients suffering from chronic hepatitis C who were treated with IFN-α. After long-term follow-up, 18 patients (21%) showed a sustained response to IFN-α therapy (normalization of serum transaminases and loss of viral RNA from serum) for up to 7 years after therapy had ceased. By univariate and multivariate analyses, no host factors were found to be predictive of response to therapy. Neither the degree of inflammation or fibrosis in liver biopsy samples obtained before treatment nor immunogenetic factors (major histocompatibility complex II haplotype and tumour necrosis factor-α promoter polymorphism) were associated with response to therapy. In contrast, viral parameters showed a strong association with response to therapy. HCV genotype 3 was found significantly more frequently in responders (P=0.034), and mean HCV RNA concentration was lower in responders (3.1 × 104) than in non-responders (2.5 × 105) (P=0.01). By multivariate analysis, both HCV genotype and HCV RNA concentration were independent predictors of response to therapy. However, exact prediction of response to treatment for an individual patient was not possible on the basis of pretreatment viral RNA concentration or viral genotype. The best association with response to therapy was found to be clearance of HCV RNA from serum 3 months after the start of treatment (32 of 34 partial and sustained responders vs 0 of 53 non-responders; P=0.001). In conclusion, determination of pretreatment viral factors, but not host factors, was significantly correlated with treatment response but did not give an accurate prediction for patients, whereas clearance of HCV RNA from serum after 3 months of therapy was predictive of response to therapy.