Measurement of hepatitis C virus (HCV) RNA may be beneficial in managing the treatment of patients with chronic HCV infection. In a phase 3 study comparing consensus interferon (IFN) and IFN-alpha2b treatment in patients with chronic HCV infection, serum samples were assayed for HCV RNA using two different assays: a quantitative multicycle reverse transcription-polymerase chain reaction (RT-PCR) method and the Quantiplex branched-chain DNA (bDNA) method. Lower and upper detection limits were 100 copies ml-1 and 5 x 10(6) copies ml-1, respectively, for the RT-PCR method, and 3.5 x 10(5) and 4 x 10(7) genome equivalents ml-1, respectively, for the bDNA method. The two assays were generally concordant over the common range of detectability. The major discrepancy was where PCR still indicated detectable virus in the sample but the bDNA result was negative. Assessment of serum samples during IFN treatment demonstrated that 37% of samples were negative for HCV RNA by bDNA but positive by RT-PCR. Differences were also noted in the quantification of baseline HCV RNA by genotype. These data suggest that HCV patients could be categorized as treatment responders by the bDNA assay when the more sensitive RT-PCR assay indicates lack of complete viral response.