Background/aims: We investigated the association between on-treatment HBsAg decline and sustained response in patients treated with PEGASYS±lamivudine. Methods: HBsAg levels were measured retrospectively pre-treatment and at weeks 12, 24, 48 and 72 using the Abbott Architect HBsAg assay in sera from 542 patients (83% Asian) treated with PEGASYS alone (180 g qw; n=271) or combined with lamivudine (100mg qd; n=271) alone for 48 weeks as part of a large multinational trial. Response was measured 6 months post treatment. Results: More patients treated with combination therapy had >1 log decline in HBsAg from baseline to week 48 ( Figure) . HBsAg level <1500 IU/mL at week 12 was associated with higher rates of response to PEGASYS±lamivudine 6 months post treatment ( Figure) . Data comparing HBsAg and HBV DNA as on-treatment predictors of response will be presented. Conclusion: On-treatment HBsAg monitoring may be useful for predicting response in patients treated with PEGASYS.