AIM:Interactions between hepatitis B virus (HBV) and otherviral hepatitis infections are well known,whether the newlydiscovered SEN virus (SENV) has any effect on lamivudineantiHBV activity is unclear.Our aim was to clarify the effecton treatment outcome of coinfection with SEN virus inpatients with hepatitis B during lamivudine therapy.METHODS:Nested polymerase chain reaction (PCR)amplification was used to detect SENV-D and SENV-Hstrains in serum from 45 patients with chronic hepatitis Btreated with lamivudine 100 mg daily for 12 mo.HBV DNAload was detected with fluorescence quantitative PCR (FQ-PCR) and YMDD (tyrosine,methionine,aspartate,aspartate)motif mutation of HBV DNA was investigated with cDNAmicroarray.RESULTS:SENV DNA was detected in 5 of 45(11.1%) casesafter 12 mo they received lamivudine treatment.SENV-Dand SENV-H were 4.4% and 6.7% respectively.HBV DNAfailed to respond to lamivudine therapy in 4 of 5 SENVcoinfected patients while only 10 of 40 patients becameSENV positive and the difference was statistically significant.Response of ALT and HBeAg to lamivudine had no significantdifference between coinfection patients and single HBVinfection ones.CONCLUSION:Coinfection with SEN virus in chronichepatitis B patients may adversely affect the outcome oflamivudine treatment. AIM: Interactions between hepatitis B virus (HBV) and other viral hepatitis infections are well known, whether the newly discovered SEN virus (SENV) has any effect on lamivudine anti HBV activity is unclear. Our aim was to clarify the effect of treatment outcome of coinfection with SEN virus in patients with hepatitis B during lamivudine therapy. METHODS: Nested polymerase chain reaction (PCR) amplification was used to detect SENV-D and SENV-Hstrains in serum from 45 patients with chronic hepatitis Btreated with lamivudine 100 mg daily for 12 mo. HBV DNA load was detected The fluorescence quantitative PCR (FQ-PCR) and YMDD (tyrosine, methionine, aspartate, aspartate) motif mutation of HBV DNA was investigated with cDNA microarray.RESULTS: SENV DNA was detected in 5 of 45 (11.1%) casesafter 12 mo they received lamivudine Treatment. SENV-Dand SENV-H were 4.4% and 6.7% respectively. HBV DNA failed to respond to lamivudine therapy in 4 of 5 SENVcoinfected patients while only 10 of 40 patients were caused to have positive symptoms and the difference was highly significant. Response to of ALT and HBeAg to lamivudine had no significant difference between coinfection patients and single HBV infection ones. CONCLUSION: Coinfection with SEN virus in chronic hepatitis B patients may adversely affect the outcome of antiviral therapy.