A striking association has been demonstrated recently between mutations in amino acid residues 2209-2248 of the nonstructural protein 5a (NS5a) region of hepatitis C virus (HCV) and sustained responses to interferon in Japanese patients infected with genotype 1b. Therefore, analysis of this sequence has been suggested as a predictor of treatment response. We sought to determine whether mutations in this region predict outcome in U.S. patients infected with genotype 1b hepatitis C virus (HCV-1b). We analyzed stored pretreatment sera retrospectively from 22 patients with HCV-1b infection who had received interferon alpha-2b (IFNalpha-2b) as part of a controlled trial. Two patients were sustained responders (SR), 7 were transient responders (TR), and 13 were nonresponders (NR). We performed nested reverse transcription-polymerase chain reaction (RT-PCR) on extracted RNA using primers flanking HCV amino acids 2209-2248 and sequenced the PCR products directly. The deduced amino acid sequences were compared with the prototype HCV-J. Isolates with four or more deviations from the prototype were defined as "mutant" type, those with one to three substitutions as "intermediate" type, and those matching the prototype as "wildtype." Of the 22 HCV-1b isolates, 6 were wildtype, 11 intermediate type, and 5 mutant type. Both of the SRs were intermediate type. The 20 TRs and NRs were distributed among mutant (5), intermediate (9), and wildtype (6). Of the five patients with mutant virus, four were NR and one a TR. Variation in NS5a(2209-2248) fails to predict interferon responsiveness in this cohort of American patients infected with HCV-1b. Thus, the utility of this sequence as a predictor of interferon responsiveness appears to be specific to Japanese patients and may reflect differences between patient groups in treatment regimens, host genetic background, or alterations in the interferon signaling pathway induced by surrounding sequences within or outside NS5a. Overall, NS5a is not as integral a determinant of interferon responsiveness as previously suggested.