Hepatitis C virus (HCV) populations in vivo exist as a mixture of heterogeneous viruses called quasispecies, which have variations in the hypervariable region (HVR). However, the relationship between the diversity of HVR quasispecies, the disease stage, or the interferon (IFN) responsiveness remains to be elucidated. To study these, serum samples were obtained from 42 patients with chronic hepatitis C virus infection;24 with chronic active hepatitis (CAH) treated with IFN, 9 with cirrhosis, 9 with hepatocellular carcinoma (HCC). HCV quasispecies populations were separated by the single-strand conformation polymorphism (SSCP) method targeted to the HVR. The patients were classified into two groups; a single-band group (n = 12) in which HVR quasispecies was homogeneous and a multiple-band group (n = 30) in which HVR quasispecies was heterogeneous. Patients with multiple bands had significantly more advanced liver disease than those with a single-band group (P = .0082). The percentage of patients with single band were 41% in CAH, 22% in cirrhosis, and 0% in HCC. Multi-variate analyses showed that viral diversity was independently related to the progression of liver disease and was not correlated with the duration of infection. We also found that in CAH, the patients who had multiple bands (n = 14) were more resistant to IFN therapy than those who had a single band (n = 10) (P = .002). These results indicate that the diversity of HCV quasispecies becomes more complex as the disease stage progresses and that CAH with more complex diversity shows less IFN effectiveness. (HEPATOLOGY 1995;22:30–35.)