We examined the quasispecies of the hepatitis C virus genome in 28 patients with liver disease of varying severity. Nucleotide sequences of the hepatitis C virus genome spanning the region from the core to envelope were used to calculate the nucleotide diversity: 0.58% 2 0.88% in 5 patients with acute hepatitis, 0.85% f 0.62% in 5 patients with chronic persistent hepatitis, 1.79% f 0.92% in 11 patients with chronic active hepatitis, 3.05% f 1.26% in 4 patients with cirrhosis and 2.71% f 1.47% in 3 patients with cirrhosis complicated by hepatocellular carcinoma. Thus the intrapatient variation in nucleotides increased significantly with severity of liver disease (p < 0.01), except in those cases of cirrhosis complicated by hepatocellular carcinoma. Multivariate analysis including the histology, duration of infection, age, sex, history of blood transfusion and serum level of ALT at diagnosis as variables showed that the histological finding was the strongest independent factor of the nucleotide diversity (p = 0.003). Serial analysis of the genome in three patients demonstrated that the intrapatient variation in nucleotides increased with the progression of liver disease. The magnitude of the intrapatient variation in nucleotides deduced from the observed changes in the patients was correlated with the mean serum levels of ALT. These findings suggest that the degree of diversity of HCV quasispecies is related to the progression of liver disease. (HEPATOLOGY 1994;20:1144-1151.) HCV, a recently cloned RNA virus, has been shown to play a major role in the development of chronic liver disease (1). Acute posttransfusion hepatitis due to HCV is followed by chronic hepatitis in more than 50% of cases at 1 yr (2, 3), and disease in 20% to 50% of these patients will progress to cirrhosis (2,4). Furthermore, a