Early changes in hepatitis C virus (HCV) RNA levels were assessed in a large cohort of African American and white patients with chronic hepatitis C due to HCV genotype 1 who underwent therapy with peginterferon alfa-2a and ribavirin in the Study of Viral Resistance to Antiviral Therapy of Hepatitis C (Virahep-C). Analyses were restricted to 341 patients who completed the first 28 days of therapy without dose modification. HCV RNA levels decreased in virtually all patients, but the amount of the change varied markedly. The overall 28-day decrease in HCV RNA levels was at least as predictive of a sustained virological response as the first- or second-phase viral kinetics responses. Factors associated with a smaller decrease in the HCV RNA level between baseline and day 28 included African American race, higher initial HCV RNA level, more severe hepatic fibrosis, and higher body weight. African American patients with similar 28-day decreases in viral levels as white patients were still less likely to achieve a sustained virological response. These results suggest that racial differences in the response to antiviral therapy are due to greater unresponsiveness to intracellular actions of interferon in African American individuals and that standard doses of peginterferon and ribavirin may be suboptimal for patients with higher body weights. Trial registration. ClinicalTrials.gov identifier: NCT00038974 .