We examined changes in the hypervariable region 1 of the hepatitis C virus (HCV) RNA that occurred with interferon therapy in 33 patients with chronic hepatitis C to assess the clinical significance of this region. The 33 patients had HCV genotype 1b and were classified into three groups based on serum aminotransferase levels during and after therapy with α interferon; long-term responders (n = 9), short-term responders (n = 11), and nonresponders (n = 13). Changes in the genetic heterogeneity of the hypervariable region 1 were determined by using nonisotopic polymerase chain reaction-single strand conformation polymorphism (PCR-SSCP). HCV RNA levels were evaluated by reverse transcriptase PCR and branched DNA probe assays. Changes in sequences were determined by cloning and sequencing analysis. Before therapy, the long-term responders had significantly lower degrees of heterogeneity and lower viral levels than nonresponders. There were no significant differences between short-term and nonresponders. With interferon therapy, viral levels and degree of heterogeneity decreased to a greater extent among long-term and short-term responders than among nonresponders. Sequencing analysis showed that the three groups had similar clone numbers initially, but long-term responders had rather homogeneous viral populations, whereas short-term and nonresponders had heterogeneous populations, but that there were no nucleotide sequences or amino acid alignments that were specific for any group before, during, and 6 months after therapy. Approximately half of short-term and nonresponders received a second course of interferon 7 to 10 months after the initial therapy; all showed an identical response to the second course of therapy regardless of interim changes in the heterogeneity of hypervariable region 1. These findings suggest that (1) patients who were nonresponders or short-term responders had mixed viral populations that had differing sensitivities to interferon, (2) the changes in the hypervariable region 1 (HVR 1) did not affect responsiveness to interferon, and (3) the lower heterogeneity in the HVR 1 was associated with a long-term response to interferon only when the viral levels were low.