The aim of the study was to determine the respective influence of pretreatment serum hepatitis C virus (HCV) RNA levels and HCV genotype on the response to interferon (IFN) alfa in patients with chronic hepatitis C. We retrospectively studied 141 patients with chronic hepatitis C included in two consecutive controlled trials of IFN alfa. A sustained response was observed in 28, a response followed by relapse in 43, and no response in 70 patients. Pretreatment serum HCV RNA quantitation with the branched DNA (bDNA) assay and HCV genotyping with reverse hybridization assay (LiPA) were performed in all patients. Seventy-four percent of the patients had detectable serum HCV RNA (43%, 77% and 84%) in the three groups of patients with sustained response, relapse, and no response, respectively (P = .005). Mean serum HCV RNA level were 1.4 +/- 6 x 10(6), 4.8 +/- 6 x 10(6), and 3.9 +/- 5 x 10(6) genomes/mL in patients with sustained response, response and relapse, and no response, respectively (P < .01). Genotype 1b was found in 7%, 47%, and 46% of the patients in the three response groups, respectively. By univariate analysis, age, source, and duration of HCV infection, serum HCV RNA levels, and HCV genotypes were significantly different in the three response groups. By multivariate analysis, the only independent factors associated with sustained response were low serum HCV RNA levels and HCV genotype other than 1b. Pretreatment serum HCV RNA levels and HCV genotype are the main and independent factors associated with sustained response to IFN therapy.