The influence of viremia on hepatic injury in patients infected with hepatitis C virus was examined by analysis of the relationship between alanine aminotransferase activity and the amount of hepatitis C virus RNA in sequential serum samples from I untreated patient with acute hepatitis C and 3 untreated patients with chronic hepatitis C. Semiquantitative analysis by the competitive-reverse-transcription/polymerase-chain-reaction method indicated that the quantity of hepatitis C virus RNA in the serum affected the disease activities of acute and chronic hepatitis C through their natural clinical courses in all these patients. The nucleotide sequence encoding the putative envelope region of the viral genome in the patient with acute hepatitis C was examined. Blood samples taken serially at 2 times of exacerbation of the hepatitis revealed 2 nucleotide mutations, resulting in changes of predicted amino acid residues. This finding suggests that nucleotide mutations in the envelope region of the viral genome may be responsible for the recurrent hepatic injury attributed to recurrence of viremia in patients with hepatitis C. From these aspects, the serial divergence of the virus genome in infected individuals, especially in the region encoding the viral envelope protein, may possibly play an important role in developing chronic infection of hepatitis C virus.