The object of this study was to evaluate the viral factor, especially the quasispecies, as predictive of sustained virologic response. We studied the quasispecies, genotype, viral load, and hepatitis C virus core Ag in 41 patients with chronic hepatitis C treated with interferon and in 84 with interferon and ribavirin. In the interferon group, responders presented a lower viral load. From logistic regression analysis of patients treated with interferon plus ribavirin, independent predictors for sustained virologic response were genotype 3a, a low baseline viral load and ≤3 bands quasispecies. Genotype and viral load presented higher specificity and positive predictive value than did quasispecies. In patients with genotype 1, viral load ≤5 × 105 IU/mL and ≤3 quasispecies were predictive for sustained virologic response. In conclusion, the predictive factors of virologic response are genotype, viral load, and quasispecies. Quasispecies did not improve on the genotype or the viral load as predictors of virologic response.