AIM:To elucidate the influence of quasispecies on virological response and disease severity in patients with chronic hepatitis C.METHODS:Forty seven patients with hepatitis C [32 with chronic active hepatitis (CAH),9 with cirrhosis,and 6 with hepatocellular carcinoma (HCC)] were screened for the presence of quasispecies by single stranded conformational polymorphism (SSCP) analysis in the hypervariable region (HVR) and non-structural 5B (NS5B) viral genes of hepatitis C virus.The 41 patients excluding those with HCC were on therapy and followed up for a year with the determination of virological response and disease severity.Virus isolated from twenty three randomly selected patients (11 non-responders and 12 showing a sustained virological response) was sequenced for the assessment of mutations.RESULTS:The occurrence of quasispecies was proportionately higher in patients with HCC and cirrhosis than in those with CAH,revealing a significant correlation between the molecular evolution of quasispecies and the severity of disease in patients with hepatitis C.The occurrence of complex quasispecies has a significant association (P＜0.05) with the non-responders,and leads to persistence of infection.Significant differences (P＜0.05) in viral load (log10 IU/mL) were observed among patients infected with complex quasispecies (CQS),those infected with simple quasispecies (SQS) and those with no quasispecies (NQS),after 12 wk (CQS-5.2±2.3,SQS-3.2±1.9,NQS-2.8±2.4) and 24 wk (CQS-3.9±2.2,SQS-3.0±2.2,NQS-2.1±2.3) in the HVR region.However,a statistically significant difference (P＜0.05) was observed between the viral loads of patients infected with CQS and those infected with NQS in NSSB viral gene after 24 wk (CQS-3.9±2.2,SQS-3.0±2.2,and NQS-2.1±2.3) and 48 wk (CQS-3.1±2.7,SQS-2.3±2.4,NQS-2.0±2.3) of therapy.Disease severity was significantly associated with viral load during therapy.The strains isolated from non-responders showed close pairing on phylogeny based on the NS5B gene,but dissimilar HVR regions.This revealed the possibility of the selection of resistant strains during the evolution of quasispecies in NS5B.CONCLUSION:Viral quasispecies may be an important predictor of virological responses to combination therapy in patients with chronic hepatitis C.Complex quasispecies and resistant strains may lead to high viral loads during therapy,with a concerted effect on disease severity.