AIM: Quasispecies of hepatitis C virus (HCV) are the foundation for rapid sequence evolution of HCV to evade immune surveillance of hosts. The consensus sequence evolution of a segment of HCV NS3 region, which encompasses putative cytotoxic T cell epitopes, was evaluated. METHODS: Three male patients, infected with HCV through multiple transfusions, were identified from clinical symptoms and monitored by aminotransferase for 60 months. Blood samples taken at months 0, 32, and 60 were used for viral RNA extraction. A segment of HCV NS3 region was amplified from the RNA extraction by RT-PCR and subjected to subcloning and sequencing. HLA types of these three patients were determined using complement-dependent microlymphocytotoxic assay. CTL epitopes were predicted using MHC binding motifs.RESULTS: No patient had clinical symptoms or elevation of aspartate/alanine aminotransferase. Two patients showed positive HCV PCR results at all 3 time points. The other one showed a positive HCV PCR result only at month O. A reported HLA-A2-restricted CTL epitope had no alteration in the HLA-A2-negative carrier over 60 months. In the HLA-A2-positive individuals, all the sequences from O month Oshowed an amber mutation on the initial codon of the epitope. Most changes of consensus sequences in the samepatient occurred on predicted cytotoxic T cell epitopes. CONCLUSION: Amber mutation and changes of consensussequence in HCV NS3 region may be related to viral immune escape.